I mean in theory if they were route cleared you could; but it would be a total waste of their capabilities.
They’re high speed EMUs with onboard power generation; for long distance express services. They’re laid out for long dwell times and long periods between stops.
Despite protestations on here Cardiff - Portsmouth is a series of medium speed regional commuter services strung together to create one long distance through service with a tiny minority of passengers using it full length of the route. The linespeed rarely gets out of the 70s and it’s heavily overcrowded so needs high density seating rolling stock and large circulating areas to reduce dwell times at stations.
Yep. Ideally you’d want something like a hybrid 387, 3-car, with through corridors, 2+2 seating, 110 on electric, 100 on diesel.
I disagree about the line speed example you’ve as these hypothetical trains are also likely to be used on Cardiff-South West and Worcester-South services where there is a lot of potential 100mph running, some quite extended. Not sure about the southern section but most of Cardiff-Bath is above 70!