Bletchleyite
Veteran Member
you say that DD trains can only improve capacity by 40%, but if a line is at absolute saturation as regards tph physically able to use the line then surely putting DD's on and INCREASING capacity by 40% should be welcomed...
A UK DD would be too narrow for 3+2 seating, regular UK rolling stock is only just wide enough for it and a DD would be narrower. If you have a simple 64 seat TSO (2+2 throughout) and made that 3+2, that's a 25% uplift already (80 seats). You can easily squash in a few more rows by using thin seat backs to get the other 15%. So it's much easier to get your 40% uplift by other means.
Buses are different in that because a DD bus doesn't have to be any narrower than a SD bus, you come very close to doubling seated capacity by adding an upstairs.
Though I do think it's poor the way we give scant or no consideration to loading and unloading times on buses in the UK, which leads to the single door layout which is hopelessly impractical for busy routes unless they are of the very traditional non-cross-city "suburbs to bus station" kind where the door is effectively only being used in one direction at any given time.