What I'm saying is that mark 1s (and maybe the early, pre-aircon, mark 2s) are (as far as I know) the only stock allowed on the national network that looks at home behind a steam locomotive. If you were to ban mark 1s from the national network* then there is nothing for mainline-certified steam locos to haul, meaning you might as well ban steam locos from the national network as well. If you want to ban mark 1s but continue to allow steam locos, you will need to construct some new stock for the steam locos to haul because there's nothing that meets modern standards which looks the part.
* and there are good reasons why you would want to
What are the particular features of modern coaches which make them inappropriate for use with steam locomotives in your eyes? Is it the length-to-depth ratio (windows in
BR▸ 's air conditioned stock tended to be shallower), the lack of opening lights, the lack of doors in the centre of the coach as was standard until the (
IIRC▸ ) the Mark 2As, the omission of the roof ventilators, or what.
If it was worth it then these features could be added to a later
Mk2▸ or the Mk3 coaches with various degrees of difficulty - with the exception of the centre door which would completely nullify the stressing!
Many, if not most, steam locomotives registered for use on the main line are already equipped with air brakes so stopping promptly has already been taken care of! There are plenty of coaches steam locomotives can haul - a lot of Mk3s off the GE lines will shortly become surplus and which could be adapted for the charter trade - all they would need is a generator van to supply the electrical train supply.
