The class 37 has proven to be the best locomotive from the original dieselisation in terms of both longevity, reliability-service interval and performance relative to original design breif for miixed traffic type 3s.
( Only time will tell if the freight only class 66 will better this from the new world of standardisation , with the sheltered lives of the Class 59s not being comparable.The class 31 and 47 failed in meeting their original design missive, both requiring expensive re-engineering. Class 33s came very close, but lack the outright pulling power in maximum and starting tractive effort that the 37 delivers)
Why did we not see more variants of a successful formula while sulzer managed to work with three different manufacturers, is lost in the past. EE had success with export and Australian manufactured 12CSVT and of course the 16 in portugal and several other countries, Rhodesia (Zimbabwe) Malaysia, and also various V8s in SVT and CSVT ( Portugal and Malaysia). The only other "variant" of the V12 was of course the almost type 3 class 31, originally a Mirlees PU machine.
However we came very near to some closely related machines: firstly it is stated that the last batch of 37s could have been "T bone" and 2000hp rated machines in 1965 ( as went to "East Africa" in the late 60s) This uprating prove to be unreliable in 37 292 and of course the BR Class 50/ D400, being compounded by other faults inherent in that locomotive. However an even higher per cylinder specific power capacity was achieved by EE australia (AEI) who delviered the malaysian class 22, with a V8 CSVT rated at no less than 1710 hp.
The v12 is a smoother engine than the 8 and 16 deriviatives because the firing cycle of the pistons is more balanced. The class 50 unveils this in the suprising "dubbing" which is the pulse of several cylinders firing in very close succession/ simultaeneously, creating shock in the engine block which resulted in failures through the 1980s. This was also seen in some class 40s, while such failures are very rare in the 12 cyclinder variant.
Given an improved timing chain, or gear driven system, and better valve gear, then the v12 in the mid to late sixties could have had a reasonable life span at 2000hp in my opinion. Of course the specific capacity of the 270 mm bore ( 10 inch bore, 12 inch stroke over square engine) went on much further, especially in marine environments. The 215mm RK215now achieves almost twice the standard traction rating of the 12CSVT.
The T bone class were a near miss, but even by the late 60s, there was marked down turn in the demand for freight and continuing electrification entailed that more type 4 , generally peaks and 40s, could be released to work the type of traffic envisaged for type 3s. The class 50s are really an after thought in dieselisation.
Back up here though! We almost got a 175hp /cylinder v16 CSVT; EE offered apparently BOTH a CVST engine for the forties in 1960 AND Co Co design to RA 6 at some point. Given these could have been made pre 1962, the loco would have been nosed, centre coded, tumble home sided CoCo. Stretched? Well not really absolutely: the portugeuse 1800s are shorter and much lighter than a class 50, being really an RA5 loco. We could have seen two variants: a no boiler compact freight machine made in the 37 body, and a dual heat longer bodied variant, both rated at between 2200hp and 2450hp, being the proven rating/cylinder of the 12CSVT from 1958 onwards.
EE could have come close to another type 3 : The baby deltic could have been a truly half deltic instead of the heavily framed diversion into type 2. In other words the 18 or a turbo 12 or 15 variant could have been put in a light tubular framed shell onto BoBo, or into a class 37 / D6700 body directly: WHy? Well why on earth did they get away with building the baby deltics in the first place? A full 18 deltic would have been fully compatible with the big sister's PUs and the CoCo bogies the same as type 3s. Lunacy? Not nearly as daft as the T9 in the bobo.
Another flight of lunacy, was the twin engined t9 deltic for both diesel hydraulic and D-E applications: the idea behind the DE version being that one engine could be turned off for hauling empties or working slower trains. That would have been a 2200 hp noisy beast.
Finally, had we seen a more rapid reduction in non standards in the late 60s, like all deisel hydraulics, then in the early 70s there may have been a gap for something standard enough to other loco fleets but of type 4 power: the last quote i have found on the 12CVST engine's rating prior to it becoming the super powered class 58 RKT, was 2500hp in a marine application, meaning about 2250hp for traction. Fitted in a shortened 50 body, or in the 50 body with dual heat , utilising an auxillary power generator, this would have been a very flexible loco: essentially a 37 with more grunt for both freight and 100mph trains. Given the superior power delivery of EE then this would have probablky outperformed the peaks and 47s.
But what could have happened with what we had. 37s !?
1) double headed high speed sub class: double heading is very standard in north america and many other countries to haul longer trains, shorten journey times and ensure higher reliability of services. This set up was tried at least three times on BR:
i) On western region, a trial in comparison / substituion to unrelaibel westerns on the highest speed expresses at that time. Deemed a failure as it wore the locos too hard for their current set up, and to do with fuel used and availability of locos to displace steam.
ii) on the Glasgow-Edinburgh service they were trialled perhaps in comparison the class 27s, end to end though with Blue Star cables run through the train. 37s could keep within 5 minutes of the heavily maintained class 47-7 route, using about 48-52 minutes on the route with the stops at Falkirk and Haymarket included.
iii) two thirty sevens were uprated to work over 100mph to propell the unpowered APT set on the ECML prior to electirfication and type 4 stock coming into use. Possibly a 37 10x and 17x. Presumably allowed to run at 1000 rpm or some electircal modification??
Given a sub class with a separate ETH geneator ( as planned , see wikipedia link to an EE document as a pdf) and four field diversions, the last coming in at about 80 to 85mph, then the syphon could have worked faster services.
The highest speed I heard was a pair working on the Great Eastern, which was calculated at 114mph, according to John a "top man" from Alfreton. Given some of the timings on the ECML and the Glasgow Edinburgh route, 37s could get up to and sustain 100mph in their original gearing.
An alternative option would be regearing to the class 40 gearing ratio, and altering the field diversion and electrical system to deliver a somewhat smoother progress for 200-400 tonne passenger trains cruising at 90 and peaking at 100mph on good sections of track. As a supplement to this option, running in pairs with the front loco switching to control would have been a sensible option for many semi fast routes.