How about the Class 373 North of London sets? Shorten them maybe and reinstate the third rail equipment :P
Aren't they being scrapped?
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The extra trains to my mind should be AT300's as that is what the class 395's are in fact with 20M carriages.
The Class 395's
are AT300's, as are the Class 800/801's SET's and the new, as yet unclassified FGW order for additional SET's.
20m commuter config vehicles are required, so it'd be more Class 395's, or train lengthening with an order for extra vehicles.
However, I presume that the AT300 would have to be supplied from Japan as the Hitachi factory here would be at capacity building both the Class 800 and the Scottish AT200 trains.
Correct. The order for what is confusingly being called the AT300's (they are all AT300's), or rather just another slightly different version of the SET, will indeed involve trains being built in Japan, due to the required delivery timing clashing with the production runs you mention.
Had the requirement been for delivery around 2020+, they would almost certainly been built at Newton Aycliffe.