As a design, the CoCo bogie has many advantages, but one challenge is getting them to go round curves and we have a lot of those in the Highlands of Scotland!
The fact is of course that Class 37s had a much longer history with the ScR than most bashers and cranks knew, although that being largely a non boilered allocation to Eastfield and Grangemouth and thus in fact, their use limited due to demands for steam heating even in May and late August on these routes.
It seems that the WHL was not a route of choice for 1960s controllers allocating out EE type 3 motive power, and I propose they were probably either a) reserved for medium weight coal and petrochemical freights elsewhere ; or b) banned from the route due to Co Co configuration.
Prior to tests of boilered EE type 3 locomotives at some point around 1977-78, I never saw myself or heard any "tractors" living within earshot of the route, nor did I see then or now any photos of 37s on even freights up there despite a decade of allocation to relevant depots. Someone will no doubt correct me and please, be my guest!
The Background History, Geography, Engineering and Economics!
By the late 70s, the mainstay of WHL traction, the class 27s, were becoming unreliable and there was a desire to have faster timetables on the west-highland line in order to compete with road transport. To put it in perspective, 27s ran a timetable almost 50 minutes longer from ft william and suprisingly 40 minutes from Oban. In a later twist of irony, these timings are similar to sprinters which required a fudging of class 37 timetables in order to look competitive.
Road improvements at various points on the Mallaig to Glasgow "A82", the road-to-the-road-to-the-Isles as they call it, were either in planning in the late 70s or actually completed. In order to remain relevant on passenger timings and also to have a synergy with the aluminum and paper industries at fortwilliam and oil/petrol to Oban, then more horse power would be needed and the 37 of course being RA5 with the pulling power (max' tractive effort) as high as many type 4 and even the type 5 deltic, would seem to be ideal (55,000lbs force at 18mph for the EE type 3)
The west highland line however is less than ideal for any locomotive or indeed multiple unit due to its torturous nature in gradients, curves and slippy rails. In steam days the line eventually demanded the somewhat over-spec'ed Black 5 be utilised, but this measure of power and superior tractive effort prove successful in steam as it did in diesel with the essentially more power than needed class 37. Many steam fans in their 40s and 50s would tell us that the black 5s would out perform the little tin-cans in classes 24,25,26,27 and 29 which replaced them from the route.
At one point there was chat and a reference in some rag about the 37s speeding up timetable and improving cornering. Perhaps that was either wishful thinking or outright propaganda then. BR was already planning for some way of working around this and although open to modification, they did not want to invest in new bogies (although at a cost we heard for a new wheel and axle with driving gears- set of over a hundred grand and this being not infrequent for Eastfield cl 37s it may have been wiser!) Perhaps someone could offer a comment on the BoBo bogies of the various type 2 designs and how the axle to axle span differs or other benefits of the double bogie with fewer axles, but that is immaterial: we were going to get more "horse" and not going to get RA6 Bo Bo class 33s up the WHL!!
They Knew All Along About CoCo Issues to be Found midst Lochs and Mountains Then?
British rail technical and the class 37 fleet management in operational engineering must have known about the potential problems of running at maximum line speeds with a Co Co loco from experience with such routes in Wales, and it is reported that some modifciations, or tweaks, to the existing bogies on ED allocated "syphons" were made to allow probably for a more compliant ride at cost of some high speed stability ( with better track bed and in particular modern points, this was probably not such an issue as it had been in late 40s BR!)
Eastfiled locos turned out with a white stripe at the tumble home bend on the locos body sides to denote this apparently, while I do not recall 37 175 having this actually, and we believed the time that 081 had it for some ceremonial/royal/tour working with an ETHEL. I do seem to remember that the white stripe denoted a speed limit on the specific locos though.
UK DE loco Bogie Design from 1947-1990 : A Fundemental Fudge ?
British locomotives like any other have vertical suspension, with the springs and dampers an obvious part of the construction. Hidden from view, the traction motors are "nose suspended" to the internal main-frame of the bogie with their own suspension such that they can ride with the axle while being cushioned from verticle shocks and horizontal movements.
Now here is the rub, you must also spring and dampen horizontal movements, because you do not want the massive forces created by the usual uneven ride of a loco as it rolls, pitches and yaws ; three aircraft and nautical terms for "shoogeling" as in Glaswegian.
Essentially this then has more to do with building engineering for earthqaukes, in reverse, where the mass which is oscillating is above the earth! There is a requirement to reduce impact forces on corners and dampen any horizontal movement transverse to the direction of travel.
So the bogies allows for this in the "commonwealth" type by having a main frame higher up in the bogie with the main circular bearing the bogie rotates under the loco on, while the axles and traction motors are then suspended on other structures which allow for some independent movement. Conversely, the characteristic 'cantilever" beam of this type of bogie then holds all three axles vertically compliant and transmits these conjoined elements and forces to the suspension springs.
So in this way the wheel set can move to follow the rail even if the loco above is pitching, rolling or yawing, and dampen the forces. Further to this then the axles and traction motors can move to a given degree of horizontal independence with the vertical forces being resolved by the cantilever beam.
As young spotters then, we thought that the cylinder and rod arrangements on this type of bogie were brake pistons, but in fact these are an ingenious part of the system to control the extent of horizontal, independent movement while actually being laterally mounted to fit in the space available to the BR gauge.
Compromise Makes for A Good All Round Bogie System in the UK:
The independent axles main advantage is of course actually in tackling curves of a radius shorter than cant of the line and wheel bevel angle would not require this (as in fact is part of the practice of very high speed trains now, where unwanted horizontal movements under application of tractive effort or braking resistance may set up oscillations, greatly amplified by the high speed: hence HS1 and HS2, bullet train in Japan and the whole TGV thing running on very shallow curves with enormous radii) The axles of a locomotive or rolling stock for that matter must to some extent follow the radius of the curves which are sharper than allowed for in the tolerances of the higher-speed-track bed solution.
Essentially despite the whole bogie rotating around a sensible designed pivot point on the loco, the wheel set when presented with a prominent curve shall we say, are on one side forming a cord on the arc of the rail and on the inside trying to form a tangent. This has to be further corrected for in order to reduce wear to both the track and the bogies, in particular the wheel flanges and tyres but also other components which we will discuss here.
The system design then for a limited amount of independent axle transverse movement has three purposes which relate in their entirety to safe progress:
1) reduce forces on the track: ie the bogie tries to educate the radius to the "chord" between contact point and indeed between bogies.
2) reduce wear on the locos flanges and other components
3) make the ride smoother and both spring load the sideways forces and dampen them to further this end.
Articulated Bogie Designs (eg Class 66)
Most north American locomotives do not use this approach of significant, radius-following, independent axle movement, but rather allow for a limited articulation in the whole bogie frame design: often on CoCo design, the innerst axle pivots or rather has a limited rotation on its own frame rather than the axles itself "floating". There are probably some unwanted effects of this under traction and breaking, or over poor points etc, and with transmission/suspension of vertical forces but now this is of course very wide spread in the UK being a feature of the class 66. For lower speed routes where it is desirable to reduce wear on the flanges and the tyres on the wheels and the axles, gears, armatures and bearings, it would seem that this would be a better solution.
However the class 37 and most of its contemporaries were based on proven technology from the 1950s and before even, from successful and reliable designs in the emerging diesel electric traction field. Ironically though, many steam locomotives feature a trailing wheel under the cab behind the main driving wheel which is articulated !!. The issue did not come up until the 37s were sent to Terra Scotia en masse! By then the articulated frame approach had been proven too at various speeds and track beds were far safer for modern locos to ride at higher speeds over.
Typical British Approach: Boffins with Cotton Reels.... Make Do And Mend with Ingenuity
Confronted with a limited budget for a limited end benefit in terms of return-on-investment then the engineers were faced with compromise and make-do-and-mend. Typical for the political atmospehere at the time, the usual public funding issue of getting a budget to repair unsuitable materials every year, but never a larger budget in one year to eliminate or engineer out those issues, no doubt played a part. 37 bogies' would then need to "shoogle" more, yet not cost the earth to do so.
I can imagine the early tweaks were then to allow for softer sideways "preload" so that the axles could react quicker and perhaps small adjustments in the total horizontal movement possible. However, a bit like a plank of wood getting nailed down really hard at one end and the other springing up, eventually you are going to reach a limit of movement in part of the system and that prove to be critical in the "nose suspended" traction motor suspension. This has both an unfortunate simple leverage effect which could amplify small adjustments in axle transversing and also of course this is the absolute key to motive power thus being expensive to get wrong.
To summarise this fascinating article as a layman; and I hope I do justice for the inginuity of the engineers back then:
- The nose suspension box for the traction motors was critical and modified to include an "X" shaped rubber suspension element to allow for more movement from more directions
- The axles were engineered or allowed to run further in the lateral direction and to do this quicker
- New side ways springs and dampers were employed to then work with this longer and faster lateral travel
The trials with the bogie were evaluated as being very positive, as detailed to a very high technical degree in the paper . The results showed that the new design conformed to tolerances for these type of movements on tracks, thus being safe and the prediction was that there would be significantly less wear on the wheel sets.
However we were in a position by then of 1985 and "sprinterisation" was planned for and the lower geared 37/4s and eventually /5s would be used on heavy frieghts at lower speeds. It seems most likely to me that there was no capital budget for this adaptation to a set of 12 locomotives or so, which seems very short sighted and a result of the whole annual budget bull shit the Uk was in at the time in all departments: on the one hand not being able to spend more on year to save costs in the subsequent years, while on the other using up as much of the budget you had for operations in order that you didnt get it cut back from the actual spend. From water works to the admiralty, this was the Tory fueled "double think" of the 80s and early 90s.
As I heard from railway staff at the time, a new wheel set for a standard 37 was over 100 grand, and maybe that was higher for the new CP7 lower geared bogie. So spending one year to save for the whole life cycle of a sub class would seem sensible. On the other hand, the actual worse cause of wear was wheel spin and some other forms of wheel-spin-prevention and arrest could have been a wiser investment in terms of cost-benefit-saving.
No matter what, 37s are still today in fact working to Mallaig after having had the longest relationship with the line of any diesel loco, spanning 3 decades.