I believe the original plan was for the 225s to be built as APT-Us in the beginning (With one power car and no tilt) as a cost-cutting measure...With the 91s eventually taken off for use with Mk. IIIs/IVs, two APT power cars being arranged mid formation, and tilting suspension being retrofitted to effectively turn them into APT-Ps.
After a couple of years experience with the APT-P trains, the idea of having centrally located power cars was dropped due to the operational inconvenience of having two half-trains with the necessity to have duplicate train crews in each half, with no connection between the two. The Intercity 225 was a compromise between the APT-S and APT-U:
Jopsuk has posted the planned formations of some of these trains: In 1973 whilst the absolutely beastly APT-T train formations were being specified for two power cars and twelve or fourteen trailers, there was also a largely ignored “APT-Lite”, with a single central power car rated for 125mph and eleven trailers. The redeveloped 1981 proposal for this moved the power car to one end of the train, basically becoming the class 91 loco, with a DVT at the other end of the train sandwiching nine articulated trailers, which was the APT-S. APT-U called for an electric power car at each end of the train, with ten non-articulated carriages inbetween (Power draw problems mentioned by others put paid to that, as well as lack of funding and perceived passenger demand).
Granted, I'm happy to accept the loss of a power car after Virgin proved that >125mph running wasn't possible without in-cab signalling (Plus the impracticalities of having the power cars in the centre of the train, no matter how cool it might look)...But I still feel let down by GNER and later holders of the EC franchise by the fact that the tilting suspension
still hasn't been fitted to the 225s over
twenty long years after they were first built!

hock::roll:

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With no realistic possibility of the ECML being upgraded to 140mph running during the 225s' lifetime, why bother installing tilt, which would be an added cost to install and maintain? At 125mph maximum, the best it would do would be to shave a few minutes off times between Darlington and Edinburgh where the route is most sinuous, but the East Coast services are already restricted by slower passenger and freight trains on this section of the route.