Alternative Type 3 That Could Have Happened

A favourite past-time of many a basher and enthusiast is to discuss the locos which could have been, or the improvements which should have been made, or if only one transmission had dominated and kept the variety of locos going another ten to fifteen years after the great deaths of 'non standards' and diesel hydraulics in particular.

We have the various incarnations of deltics, from a super syphon with a turbo deltic T 18 for rattling up the west coast main line in the mid 60s to the super deltic working in pairs  on god knows what ever train would need 8,800 hp. We have then the sulver variants, the LD8 with intercooling and the LV8 which was used in some French locos. Then we have Mayachs

Yes indeedy it would have been fun to have a type three with a maybach in it. We had of course the Hymeks and tantalisingly we could have had a real beast if they had dropped the boilers from Westerns and fitted with twin MB870s as used in Hymeks instead of the v 12s. A single engined diesel electric version of the hymek would have been interesting indeed.

As we can judge from the BoBo diesel hydraiulic, which was given a route availability eq to RA6, a traditional build requiring boiler space would entail a diesel electric having six axles, most likely being a CoCo but since this would have been made somewhere quirkly with either Crompton or Brush or maybe GE electrical systems in the early 60s, it could well have been an AIA configuration.

How hell fire would it have been? Well oddly the Hymeks were arbitrarily depowered just for the purpose of being a type 3, incidentally the same fate falling on the Brush type 2 when they went v12 svt and lost about 45 hp to come under the type 3 threshold. type three was  not up to 2000 hp in the BRB rating system, it was medium power from 1500 to 1750, wiith type 4 being from 2000. The little gap meant that the standard rating for the EE CSVT v12 and the Maybach v16 was dropped. EE had exported two to three versions recognisable as Class 37 cousins by 1960 with 1850hp and the standard traction rating in the v160 series German locos was 1940  horsepower for the maybach unit. Ah the oddities of pipe smoking beaurocrats with public school accents, sticklers for rules in black and white.

In case you didnt know the germans had a crack at a twin v16 engined beast , the v320 which nearly became a class., In their wisdom and over zelousness to be marvellously good at service engineering, the loco survives today, earning revenue on the DB to 2009 from its launch in 1962. It is a monster, sounding like a pair of thirty sevens on crystal meth, you can here it best here https://youtu.be/vFqI27GQbzY .     Even a single of these MB839s as they were then (maybe a twin turbo predecessor of the 870?) would have made a formidable loco at the foot of type 4 rating, 320 001 being rated at 2 x 2,000hp.

Even at 1750 hp the Hymeks found themselves being selected for fast medium weight expresses on GWR and by many accounts performed well and were more reliable than their twin engined oil sloshing cousins. Of them all they really semed the most sensible to keep on operating with, and there could even have been an attempt to convert them to diesel electric since they have such standard body and bogie construction. Alas the pukker little type 3s met the same fate as all else fitted with a big torque convertor.

Hymeks could have lived out another two decades in the far west country or wales, given boilered stock survived into the late 80s and they would have been converted to air brakes. ETH would have been a hard ask because there is no drive take off on the other end of the crank. A full DE coinversion or a sister class with CoCo set up would perhaps not enjoy such utility from the 'high' speed power unit. It is hard for the lay man to tell, but of course so many DEs now are running at 1200rpm or more now! Perhaps there would be some electric field benefits of the engine being able to run at either a longer first field until diversion kicks in, or more importantly a longer second field than 37s did, them dieing on trains which had to run at 40-50 mph banging in and out of weak field. Or like the v12 early 70s HST power cars, perhaps there are far smoother transitions with a higher speed engine matched to its direct drive gernator or alternator (??)

There is  a great irony in thinking of a DE TYpe 3 or 4 with a marvellous Maybach v16 nestling in its boxy interior, and the demise of diesel hydraulic locomotives en masse, Today's British Railways depend largely on hydraulic transmission for most all classes of DMU, while of course HST power cars have been in part re-engined with you got it, v16 Maybachs! And more reliable than their VP185 multi turbo compeition they are allegedly.

Locomotives In a Spin> High RPM and Multiple Power Units versus Mid Speed

It is interesting that some things return full circle to the way engineers thought in the late 1940s and into the 1950s when presented with a problem to solve> Back Then it was replacing steam with quite literally,. a turn key solution, while now it is replacing the second generation of 1970s&80s Diesel locomotive with locomotives which can comply to the outrageously strict emissions and noise legislation laid down by the EU commission and also lead by California who have pointed their judicial compliance bow to zero emission locos. This in itself is a farce becuase even diesel loco hauled trains are many hundred times more efficient over any long haul route than the equvalent 20-50 lorry loads or 200 - 300 personal car journeys. Rather than painting power units in locos with the same black brush as mass anarchy transport, they should be setting a sensible level of progress on these fronts, all be that very much quieter locos than we enthusiasts may like to hear pounding the metals..

Today Seimens present a modular power unit loco with possibility for dual power (over head or maybe third rail units top be popped in) and / or battery packs. One key benefit they quote for today's operations is that the locomotive can dial in and out power as it needs it, with the power units then working at their most efficeint peak range more of the time when they are on, thus reducing fuel consuimption and emissions,. In their Marketing PR launch discussion they talk with no reference to the long history of multiple power working doing just this, dialing in more power when needed, while then saving fuel by cutting back on parts of the route which do not need so much horse power. Rail actually requires a large tractive effort to start a train and to take a train up a steep or continous gradient, but since the days of Rocket it has been known that there is a lot of coasting and low tractive effort haulage going on due to the inherent efficiency of running on rails, particularily with roller bearings and optimally loaded axles. Thus you are actually dialing in power a lot more in a train than you are in a lorry which has a lot more relative wind and rolling resistance than a train, and you can times that by as many as 60 in europe for the biggest 2000 tonne trains

I can't remember if the PR release mentioned engine redundancy in case of power unit failure, but this has to some extent been quoted as the reason for multiple engined locos. This may have been at some point someones 'also ran' selling point, be that point made by a supplierr on internally to British Rail western regiona and the Deutsche Banen in particular. In fact the notion is largely a red herring : in the case of the German multi engined V200s/220s and the British Deltics, the attraction was more horsepower per se from a single locomotive within some technical limitations. Interestingly those limitations were quite similar and both technical. In the 1950s there was a desire for light locomotives with 2000kw output approximately to sustain speeds of over 90mph/150 kmh and run ideally cruising at 100mph//161kmh.  The limitation were on both diesel electric and the hydrualic mechanical transmissions at the time.

Both systems were limited to about 2000hp per power unit by technical limitations. Firstly in the electrical generators, where English Electric had a size limitation and a knowledge of flash over (which would later plague the class 50 and competitor's class 47 locomotives ) in the dirty railway environment for the then DC generators. The biggest EE could offer was about 1.75 Mw , brush and AEI offered slightly higher while the American locomotive manufacturers could offer their home market around 2.2 by the early 60s with the larger loading gauge. The DB in Germany had opted for Diesel Hydraulic for their higher speed services and much of their other locomotive provision in the 1950s. The limitation to them was that at that time for the footprint required at least, neither Voith nor Mekhydro could offer power transmissions of over about the same 1.7 mw, or actualy at rail about 1500 hp.

One efficiency advantage of diesel electric in this respect is that a single engine can be employed to deliver the maximum applicable kw to the traction motors at start and low speed in a light weight express locomotive. In the Deltic this means that the power control handle does not activate the second engine until approximately 18 - 22mph is achieved. With the secondengine revving up to deliver power to the same level as the first and then both going onto rev further to deliver 50mph 
, the system as a whole is very efficient for passenger workings because they require exactly this type of acceleration avoiding a field diversion(electrical gear change). A typical 350 tonne passenger express of the late 50s'1960s requires light starting effort and offers actually not very much momentum to push through field diversion  . This explains why the class 50 out accelerates a deltic to over 30 mph, while a class 37 will out accelerate a deltic to 18pmh hands down because it can lay down far higher amps as its single engine is delivering maximum mechanical effort in the first field which dies at around that speed. Both these classes however could be plagued at speeds of under 50mph by trains not having enough momentum to push through the diversion, then meeting a gradient, or by a speed limit or signalling which meant the engine hunted between two gears as a manner of speaking about the automatic detection and switching equipment. The twin engined deltic and presumably Brush "Falcon" overcame this issue by dialling in the power and avoidimg low speed diverts, while the former offered quite low starting effort and maximum continuous effort due to having a lower amps/ higher voltage system (amps are a measure of torque where as higher voltage relates to spinning if you like, groossly over simplified) 

Diesel hydraulics however have a different advantage in having their torque convertor 'half gears' ie slippage in the fluid coupling followed by the married phase, as well as having a range of 'hard ' gears to change up to.  Correctly engineered this makes for a very smooth acceleration with virtually no wheel slip and quite a high starting effort. Both engines can power their respective bogies from a standing start without the typical electric overload of 1950s first generation DE locos, they are designed to be geared correclty for this tractive effort and the limiting factor is how long you can cool the hyrdaulic  transmission oil when it runs in fluid unconnected phase (slipping stator relative to rotor). For all types of trains in fact, a twin engined diesel hyrdaulic has another slight advantage over DE designs of the late 50s at least, and that is that one complete transmission bogie system can be left on maximum power while the other system reves down and changes gear for its next bit of the cherry so to speak, thus the train can be kept at a constant speed if not on a steep gradient. Field weakening on DE is a just a natural physical barrier for the entire system if it is to run smoothly, although a complex out of phase double system coudl be concievable, where the two bogie/traction motor sets are tuned to different field weakenings. Instead as in the deltic, the number of gear changes to 100mph is just three versus effectively six for diesel hydraulics even when they are more powerful as in the german V classes. 

As touched on in the pre amble above, the redundancy of twin engined locos was a secondary selling point in effect then. However it did mean that the most important express services on the GWR and the ECML could limp home at about three quarters speed in the mid 1960s when the whole genre of diesel was still to be proven as a mass produced item. Further into the 1970s, the HST IC 125 sets had two power cars in order to deliver the magic extra thousand over deltic services to go 125 mph., but the betting on effectively two power units per train proved prudent in terms of limping home as both the Ruston Paxman valenta and the Mirrlees power units prove to be less reliable than anticiapted for their service intervals in the reality of the dusty conditions and high thermal cycling loads placed upon them by the nature of having more stop starts than originally discussed with the manufacturers. The 125 was actually designed with train crew changes en route and fewer stations than the deltic and class 52 services they replaced. 

In terms of fuel efficiency then I have not heard of westerns warships havbing one PU shut down, but it happened regularily on Deltics which would have their second engine shut down on the slower Ediinburgh-aberdeen section of the expresses from KX. Presumably after ETH was introduced this required that the second PU be on a heat only selection, revvving not far boave idle I do not know. Deltics did limp home and run light loco on a single PU, and especially with the teething problems with pistons they encouuntered, this became a very fortuitous feature.

In post war Europe and the UK there was still materials rationing and oil was imported from the US and Middle East, so the use of multiple workings, dual or more locomotives is for those historical reasons far less wide spread than it is in the USA/Canada, coupled to a steady decline in freight from the 1960s to the 2000s as the road network improved. The states on the other hand and canada had a heavier loading gauge and bigger, more economic commodity and produce loads to haul by rail and thus it was economic for them to run multiples and 'robot'  locomotive arrangements were widespread by the early 1960s with multiples of up to 4 at the head and more down the train. I do not know of any european robot multiple locomotives apart from shunters like the class 13 and some on the continent, robots being a unit which lacks a proper cab and are only used in multiple with cabbed locomotives leading. By 1960 the BTC/BRB had abandonned through connection doors for locomotive design, although many were in still in production then, and were stipulating that type 4 motive power be above 2700bph and not fitted with multiple working ( a decision reveresed in the class 50 due to the gradients of the WCML and the ambitious timetables laid out in the run up to electrification in order to keep the route competitive in respect of the new M6 /M74 and the advent of the Glasgow Airport-London shuttle. Class 50s were not fitted with these as a response to theirr reliability issues, which were in fact somewhat better than their rival mass priduced brush type 4, which required a massive rectification at a cost of over three million pounds back then. That class 50s were delivered without multiple working cables t first has been cited as being because of availability of the materials at the time, safety testing not being complete, the price of the class 50 running into problems and then this being taken as part of the hire-purchase scheme they were bought on, subject to absolute rquirement for the faster timetables north of Crewe and later Preston)   

So at the end of the day you have to examine what your actual missive for tractive effort and maximum speed is, and in fact this is where DE wins over DH:  For the same horse power and power unit arrangement, " Faclon" was a better performer than the class 52 Western hydraulics with their voith systems. The limitation here was that the v12 MD655 engines were not powerful enough for the  third gear on the Voith transmissions which were presumably very similar to those fitted in the more powerrful German V2xx locos. It may have been possible to have fitted tiwn v16 MD870s or alternatively a Mekhydro gear case could have provided a better matched power deliver for running at over 75mph. The Voith transmission is in theory smoother than the Mekhydro due to it working on a priciple of triple, sequential torque convertors and hydraulic actuation of these three main gears. Although Westerns and of course Hymeks saw a lot of working on relatively heavy freight services, the similarily powered Falcon and the Class 37 EE type 3 both out performed their respective DE cousins in standing lift and progress, all be that with some degree of wheel slip on the EE type 3. Deltics did work a very few freights in their lifetime and heavier night mail/sleepers of over 650 tonnes, famously last year D9000 working some heavy lifts to the rail head due to lack  of class 66s.  Falcon, the class 53 one off from Brush, excelled at both freight and passenger working, and indeed ended its productive life on ore and coal traffic, slugging them around south Wales. Class 50s were designed on out set to be a stop gap passenger loco for the WCML but also then to have the possibility to work higher speed freights of upto 1000 tonnes, being in fact prepared with mounting points for buck eye coupling as if it was going to happen in the 1970s.  

We have then come full circle in which transmission system suits which traction missive: For light passenger trains of 40 mph to 80 mph the vast majority of diesel services in the UK are now sloshing oil over convertors with mechanical hydraulic tranmission. This was a decision taken a very long time ago by some engineers, probably ironically enough, about two or three years after Western Region lost their last main line DH locos. For heavy freight you want to slug it out with 25 tonnes per axle at least and over 3000hp in DE, while also the hands down winner for mixed traffic is the DE too because of the controllability and range of electrical traction systems. This is reflected in the delivery of the latest class of locomotive to the UK, the mixed traffic class 68 which is pretty much the type of locomotive the GWR would have ordered if they had been forced to run with a larger proportion of DE locos. The main point lacking here is that we do not see many multiple power unit locomotives with their specific advantages and redundancy, but that in main part is due to the amount of power available in reasonably sized single power units in mid or higher speed * GM versus Caterpillar, MTU and the most powerful per weight, the VP185 from Rustons /MAN, and also that there has been no specific missive for this type of locomotive. Even in DH we have seen several Vossloh locomotive classes and the Anglo Belgian powered freight class emerge with only single power units. It seems the reliability and modern engine management have by in large rendered twin engines obsolete, where as the Seimens loco can carry up to four power units.

Mekydro and Personal Enlightenment on the "Oil Sloshers"

Type three locos are a bit of a dark eve's obsession for me now, as much as chasing 37s about in the 1980s was then.

The one I regret not being in service when I was a nipper, was the BR Class 35 Hymek. With its' tractory, thrashy v16 single engine and petite and pretty looks the loco design seemed to punch over its weight, and many a Hymek nut who worked on them in service life, will tell you they were the best Diesel-Hyrdaulic Locos in the UK.

I was delving deeper into the Hymek story when i actually fell into a great disappointment or rather a hole in my knowledge due to being a presumptious of understanding, runs in the family, son #1, 5.6 y.o. bullshits away to me about how things work or are too. I happened upon a set of GA drawings (general arrangement) for the whole loco, and the bogies. It struck me that for a large part of the last four years I have been going over GA drawings being a technical outsourcing purchaser , and that I had actually bought some diesel hydraulic off the shelf equipment: a rotary table with turbine motor run from the "main circuit" in turn probably run directly from a diesel unit like a v16 cat or a rustons v12.

So I decided to cast my eye more critically upon the drawings and saw a "whole bunch" of cardan shafts. The Hymek type 3 has of course only one power unit so must distribute the drive to both bogies unlike most other DH locos of the time.

It dawned on me that I had gone along in a little myth about DH locos which was a self told fallacy : I had thought that the locos used a gear box and torque convertor to pump oil into what we call a hydraulic motor or turbine motor at ever higher pressures as speed went up, with hosing going to each driven axle and a final drive turbine motor arranged around the axle. Well the name is Diesel Hydraulic ???

In fact though I now discover to my ignorance, that the term Mekydro coined in good old Deutscheland is far more accurate: Diesel hydraulic loco is a misnomer!!! These locos are mechanical with hydraulic torque transmission.

Given a single torque convertor in fact, the amount of oil in the system would be probably no more than in an equivalent big mid speed engine like a 12 CSVT or 8LDA of around the same output. You of course run the fan and you need to have a volume and cooling heat exchangers to suit the thermodynamics of the oil so that adds, but the point is that a single torque convertor transmission for a v16 MTU today would be relatively small, with a matter of tens of litres of circuit volume for fluid in action so to speak.

So anyway, self made mad myth busted ( unless someone tells me the final drives are cardan shaft driven turbines or fluid mechanical gear joints), I started thinking more about how these things work in practice and what the issues are with them and what the advantages over diesel-electric are or other tranmission forms?

Why are DE and DH Favoured over Mechanical Geared Direct Drive in almost All Locos over 1000kw ?

There are several shunters, light self driven rail cars or sets (DMUs) and of course the whacky Fell locomotive of much more horse power,  which use gearing and clutches to achieve transmission. In 1987 a pal of mine who was technically minded asked me why there were not more mechanical direct drive locos and why they resorted to DE or DH drive....

The main issues in using direct mechanical drive in a loco, from the type you would find  in a car with a standard clutch or a motor bike:

1) Wear and tear on gears and clutches, due to high starting loads, variable speeds, and uneven travel
2) heat in the gears and oil
3) requirement for a great many gears for higher speed, higher horse power to reduce and apply the power successfully
3) difficulty in starting heavier trains

The last two matters really hit the head of the nail, whereas the first two are actually problems partly shared in DH locos because they are mechanical-hydraulic drive with basically the clutching of main engine drive being the hydraulic bitty.

In order to start heavier trains , just as in slipping the clutch on your car on a hillstart, you need to be able to exact a stationary starting force on the axles in order for them to have enough torque to start to rotate.

A bit like getting a nut opened with a long bendy spanner, you need to ease in the power without it :

1) on the one hand resulting in instant slip as the right pressure is met and then quickly exceeded by the leverage of gears on the axle,
2) on the other hand  that there is deformation of the gears or excessive wear on the clutch plates.
3) Pull a third hand out from paddling kids at Sellafield: You need to change gear immediately after you start as the gearing is so low to achieve this "bendy torque wrench" effect, thus making progress awkward and potentially stalling the train anyway.

Here is where DE and DH locos excel and why the first series of uk diesel multiple units were erm, a bit crappy. They can both start a much heavier train and retain reliability and sensible construction of the transmissions.

How the MEK is Attenuated Nicely by the Hydro

As I realise to my own chagrin, DH is an overstatement: we are talking an automatic ford granada is technically spot on compared to the turbine breaks of the APT-P.  Now there happened to be a lot of old Granada Automatics in the  UK in the 80s and 90s and of course Americania lasts for ever from the fifties and sixties (before they got too fancy and all gay with their trann'ies pun, pun) so in fact the system would seem to have some longevity benefits.

Usually DH locos have a transmission shaft "stator" array which passes through several turbines , ie the donut shaped torque convertors themselves. The torque convertor looks a bit like a triallabite but it is actually a marvel of simple, wonderful engineering. It allows basically for a slipping clutch without any mechanical wear on its internal exchange surfaces unlike a standard car or oil encased multiplate oil clutch, the motor bike type, which are incidentally common in machinery. This slipping of the clutch is effectively a form of variable gearing allowing for both

a) a standing force to be built up progressively while the drive shaft is loaded up with torque smoothly
b) the engine to carry on to apply more power from its own torque output , the curve of which may be a lot higher than a direct mechanical system could take, thus burning out a clutch slipping.
c) Effectively this is a continuosly variable gearing until the rotor speed matches the stator speed at which the application of torque becomes linear - until one or the other slips -
d) coming back on that point in c- this also means that there is an inbuilt resistance to slipping in the convertor as the blades do not want to pump oil backwards but rather the system will slip  if the speed differential becomes greater than the designed flow.

Now here is a great claim in C and D we must come back to

The best and worst is really to be heard in the sprinter units which have fairly small engines which have to be revved hard to get the starting force going, and then there is either a gear change or the engine backs down to allow for synchroyny in the torque convertor, which it sounds like it does and feels like from the speed being fairly un -sprinty.

We have looked at a single torque convertor which would be fine in itself: this would in effect mean that a locomotive with an engine rpm range effective of 700 rmp engaged to max would run a single mechanical gear much longer than if it was directly connected to the output drive shaft, ie the clutch was let out and stayed on. You would need more gears to cope with the torque differential. Here also as you go from a starting torque and reach maybe synchrony in stator -rotor speed which should in theory be linear, a peaky v12 say can continue to exploit the slip of the clutch in applying its own progression of rpm and torque which the wheels catch up at the end of the day.

Back to sprinters then: horrid things, they rev far too much for the progress the rather light train should be making. The 170s and after comers, are a bit better at applying power though but still irritating compared to a loco up the front. The point being here that if you have a revvy, low torque engine you have to rev a lot to get the starting force out, noisy, vibrat'ey, and then you have to change gears a lot at lower speeds to continue to overcome the inertia of the train until the resistance is then more friction: 170s are pretty quiet when powering along at "high" speed.

The irony of these horrid plastic rail-bus thingies,  ie modern DMUs, is that the majority of non electrified routes are now DH driven! How a few long gone GWR traction engineers must have sniggered in oily-sloshy heaven when that day came in the late 1990s.

On a more powerful loco then you have a lot more torque to play with from a big engine but you have then a lot more torque to control and apply and more potential friction in both mechanical components and in deed on the hydraulic oil.

You need gearing but by in large that is limited to three forward and a reverse ( I wonder if they have a heavier system to change direction of travel so that the gear direction is fully reversed? ) In effect your torque convertor is also giving you an extra low first gear without a gear change to second slow speed acceleration gear, and then onto a third.

However the clever Gerry Clogs at Voith a very long time ago now, came up with the idea of multiple stator-rotor turbines arranged along the same shaft. You then engage these by hydraulic actuation of each circuit with it being essentially on low pressure, lubrication when the circuit does not pump oil in. This means that you can probably then (?) rev the engine down less to change up a gear because

a) you can slip out of gear and into the new without major issues in just engaging the new donut down the line and phasing out the current gear
b) the new gear will be able to catch up by the very principle of torque conversion.

The sequential convertor gearing, three stages of them on a Western Voith transmission, are then based on delivering progressively higher pressures to the rotor drive shaft or would that be higher flow at lower pressure? looks like they go down in size like mechanical gears and that also the power relies on a longer smoother application of torque /rpm from the main mover Power Unit.

Now anyone who has driven a modern lorry over 10 tonnes will know that they have a lot of gears, and you use most at low speed, as above noted for DH drive. Another approach is to have more gears on a single torque convertor.

The benefit here is that you reduce the complexity of the hyrdaulic transmission and control, while also you can have the gears in a smaller volume per gear because torque convertors and their associated hydraulic control and cooling equipment take up more space than gears with multi plate clutches.

I think if I remember right, the warship class had each mechanical gear case on the engine side of a single torque convertor, whereas the Hymek have their single gear case after the convertor. You introduce then the need for mechanical - hydraulic feedback control for speed in order to shift gear automatically and not have the driver doing it (crunch, screeach, kangeroo starts and so on....safer to give them a simple couple of levers to pull on and let t' loco look after hesself)

Also in having three or more gears in a mechanical box, you introduce more points of maintenance and more parts which wear out. Torque convertors of this type cannot run in reverse and in fact the stator is locked in a single rotational direction to ensure this! Otherwise pressure differentials could brake and reverse the stator potentially

Surely a better compromise would have been to have a twin gear hydraulic torque transmission and two more gears in a gear box with also the main direction of travel ? Or maybe an inboard gear box, two gears, then the double convertor system and then a "reverser" ? Essentially then you get a very broad range of operation speed-tractive effort with a combination smooth progress when you most want it, over then efficiency when you switch the big mechanical gears? Also in that arrangement, you reduce the opportunity for heavy mechanical forces being applied to the main gear box, while also making that gear changing box simpler by taking out the reverser? If you do get some kind of damage caused by a nasty sheering force ("negative torque" torquing back to you so to speak LOL ) or when the loco crashes or jolts its bogies, the simpler reverser gear box takes the hit and is designed to break somehow limiting damage "up river".

Advantages DH vs DE

If you could have had greater reliability in the higher speed power units necessary for DH in the 1960s then you could have a big advantage over DE straight away because then your service interval is longer for major out of traffic overhauls, and given there is no damage, the worn parts can be changed out quickly. The parts list is allegedly cheaper than DE of the time of big dieselisation.

The other big advantage which is alleged is that DH designs are very resistant to wheel slip. I guess this is for two reasons- firstly there is a lot of built in inertia and friction up line in the carden shafts and bevel gearing, and secondly because the torque convertor is both locked in forward direction and the rotor will encounter resistance if it tries to go faster than the stator. Unlike in a car, the engine will not be dragged into the wheel slip by suddenly being free to rev a lot faster.

In older DE designs, wheel slip had occured before anything was done about it, and in the lighter designs or those like the 58 with poor bogie design, the rate of damage by wheel slip on steeper routes must have been frightening for the depots accountants.

The jury is a little out on the wheel slip thing. It seems maybe that electric traction motors can apply a far higher starting force and a higher / faster application of torque through the speed range than hydraulic locos, and that a lot less power is lost to heat ie they are more efficient at converting power even though they go through a tortuous mech-electric-mech route rather than the oil sloshers more direct path.

The wheel slip advantage is something the jury is out on with the new Voith loco 3.2 kw jobbie DH, erm , gaining traction in the market. However given you are allowed to run bloody heavy stuff like 66s and 60s and the new electric creep control and wheel slip avoidance / detection control and SEPEX not in the least, then the benefits of DH are diminished.

Disadvantages of DHs as built in the 1950s / 60s in the uk are

1) engines can be thristy relatively on idle
2) both the engine (power unit) , gear box and especially the hydraulic transmission are /were prone to overheating
3) General Heavy Repair can be very costly requiring many new gear parts

If the fleet of DH locomotives for the GWR were for some reason, out to tender today and not in 1958, then we would see of course the use of computers and  a lot more experience in building and operating them in Germany. Then you could optimise the whole system for a given use, tonnage and speed range and keep it within its boundaries, while having very very efficient gear changes due to computer monitoring and control. Also as has been recently obvious, 1500 rpm engines in the 1 - 2.5 kw range are seemingly necessary in order to meet the new emissions standards, and GE / Catipillar boast long service intervals on these units. Finally materials, machining tolerances, lubricants, and of course hydraulic oils have all become far more advanced since 1958.

So today the GWR would get an interesting bunch of locos, probably including Voith's own monster and some Vosloh "dog bone" locos which would no doubt compare favourably to the class 70 in particular for freight work, and be a far nicer way of applying oily mediated progress to passenger trains than those rather horrid 3rd gen DMUs.