There are also already a load of Mk3-based "DEMUs". They're called HSTs, and they are going to get tarted up and deployed on Scottish InterCity services for many years to come.
Having 9 carriages and 2 power cars makes the HST's (much as I love them) not really suitable for most of our routes. I don't think that Scottish Inter City services will by any means keep all of them occupied, and that's without considering all the Mk 3's that aren't in HST's and will be unoccupied.
As for our 442's, I know it's been done to death, but would converting them be any more peculiar than converting an underground train ?
156s are not suitable for commuter services, having an end-doored layout. They are fine for branch lines and for regional expresses, but that is not the only requirement for rolling stock for the North.
I think that the end door thing is very much overstated.
As someone who uses busy trains in the North a lot, I end up getting on and off a lot of very crowded 156's, 153's, 155's and 144's/142's (I know the pacer carriages are shorter, but there's only one door for both the inner carriage ends, so these are nearer to the end door layout of a full length carriage than suburban layout). I don't think the difference in unloading between those and a 150 (the only diesel units in the Northern area with a suburban door layout) is that great.
