I’m not sure that the ECML electrification project was anywhere near as advanced as some people assume; at least beyond the London commuting area.
The
Modernisation and Re-equipment of British Railways included electrification of "Kings Cross to Doncaster, Leeds and (possibly) York" (para 43).
The
System of Electrification for British Railways was first published in 1956 with a revised second impression in September 1960. This also included electrification of "Kings Cross to Doncaster, Leeds and (possibly) York" (para 111).
So what would have happened north of York? Was the electrification to have stopped there and a Weymouth-style arrangement handled trains to Newcastle and Edinburgh (assuming the route between them stayed open)?
Or was electrification to proceed beyond York and a mix of 309s and 81-85-type locos to be used?
What became the Clacton EMUs were ordered in autumn 1960 and reported in
Trains Illustrated December 1960, which said:
The Clacton multiple-units CONTRACTS have been placed for a series of new 25 kV a.c. electric multiple-units which may be prototypes of the kind of 75 m.p.h. average, 100 m.p.h. maximum trains we were promised in the 1955 Modernisation Plan. They are 15 four-coach and eight two-coach units, including refreshment facilities, for the Liverpool Street-Clacton and Walton service, the two-coach sets to permit ten-coach formations in the peak periods, and they are due to begin operation in 1962. The units are deliberately provided with more power than is strictly necessary for the Clacton service so that they can be assessed with an eye to even faster multiple-units for long-distance work in the future.
Gerry Fiennes had given a presentation to the
Railway Students Association "The future of railway passenger traffic" which was reported in
Trains Illustrated February 1960. At the time Fiennes was Traffic Manager Great Northern. The presentation gave a hint of what an electrified ECML passenger service might look like viewed looking forward from 1960.
As to frequency of passenger trains, Mr. Fiennes held that on the principal routes, except at night, nothing less will suffice than a half-hourly service up to 70 miles and an hourly one between 70 and 300 miles. Since the pattern must give services not only between London and each principal town on the line of route but between those towns themselves, either the main line expresses must make fairly frequent intermediate calls or there must be a closely linked system of feeders; or, more probably, a combination of both. Thus the Great Northern plans an hourly group of three expresses respectively to Leeds, Newcastle and Sheffield/Hull, linked with feeders at Peterborough and Doncaster or York.
A plausible interpretation of all of this is loco hauled hourly fast trains with "feeders" formed by something like the Clacton EMU.