I suggest you read this: https://www.railforums.co.uk/threads/class-730-lnr-wmr-delivery-test-updates.215134/post-7239159.Northampton - Birmingham really needs 12 car trains but some of the platforms are too short. A possible solution could be too extend the Litchfield Trent Valley - Birmingham International local trains to Coventry. The LNR Euston - New Street services could be extended to 12 car trains and not call at the smaller stations between Coventry and New Street.
Yes, that's the plan and alongside that is the plan to run 12 car 350's between London and Birmingham New Street using the surplus units no longer needed on the Trent.
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Lets not forget that the plan for the 730's was at least DCO if not DOO instead of the conventional working the units have needed retrofitting for.
The 350s have UDS which allows them to release the doors in the front 8/4 coaches on short platforms, so it shouldn't be an issue. There is a programme to retrofit them with ASDO, but whether it will be used or not is unclear.