Good question!
I would imagine that by the time that BR designed the MKI stock, the need for more suburban stock dictated a standard length based on the space required for a compartment. LHCS coaches inherited from the big 4 companies were generally shorter than 57ft and given the shortness of suburban platforms at nationalisation, 57ft with nine compartments was a reasoanble compromise. When the MKI mainline corridor coaches were designed, there were also the programmes of EMU building, late 4-subs, 4-EPBs and the GE/LMR designs which ran high density services where platforms were generally longer, so there was an opportunity to create a 10 compartment underframe which neatly fitted under a 3+4 composite corridor coach with gangway connections and toilets, also it suited the TSO layout with 8 bays with toilets. Thus the suburban 57fts continued to be made as it must have been obvious by the Modernisationn Plan (1955?) that they would be the last non-corridor LHCS requirement.
With a compartment length of about 6ft, a 51ft pre-war coach could accommodate up to 8 compartments, a 57 ft nine and a 63ft ten, (which is why bullied went for 63ft, - see my post above).Glad you asked that question
I don't know the answer (someone else here probably does!), but coach lengths were certainly not standardised pre nationalisation: both the LMS and GW pre-WW2 built corridor stock to 57' (though not exclusively), the LNER 60' and the SR 59' - the GW also had a fair amount of 70' stock including suburbans.
Post-war, Thompson and Bulleid both went for 63' while Hawksworth moved to 64'. Bulleid at least produced 63' suburban EMUs.
Did this coincide with improved manufacturing techniques, I wonder? ...
Much of the LNER suburban services were run with 1923 designed 'quad-arts' which in terms of passengers per length were quite efficient owing to the smaller gap between coaches. The clearances on the suburban lines would have evolved to accommodate them and until the the widened lines were electrified in the early '80s, the 57 ft BR stock would probably be the only stock that fitted (just).
Other way round - 57' preceded 63' consistently.
I'd wondered about traversers too but I don't think any carriage works date from the 63ft era.
I think that's true, but the MKIIs were of integral construction so the concept of a common chassis no longer applied. That meant that the length could be adjusted to provide additional crash resistance in the superstructure.
There may also have been an issue with resonant frequency.
Even then they were tight, only 317s/319s and 31s being allowed I think.
Apart from 31s, which don't hold many passengers, those classes weren't in existence in the '60s.
No indeed, but the post I quoted made clear this was after electrification.
I didn't dispute that, but never mind.
Was the LNWR at Wolverton that finally reached 57ft as their standard length, LMS inherited it from them. Think the coach departments of midland and LNWR were the only sections to actually merge without argueing or requiring a dominant partner, so 57ft was accepted by all. The new all steel underframe was the deciding factor for the length. Partitions within adapted to fit as required for the different configurations - first class, second class, composite etc.
