Southern Region's 4SUBs and 4EPBs had a door for every 10 seats, and the 4VEPs were almost as well equipped - one coach in the set had eleven doors per side. Of course they were single doors, but compared with two double doors per side on most modern stock, which on some trains are quite narrow, those older trains enabled large numbers of people to get on and off quickly, and people were prepared to "move down inside the car" because they'd never be far from a door.
Nowadays people are reluctant to move down away from the doors, especially if they want to get out at an intermediate station, for fear that they won't be able to get past other standing passengers before the doors close.
But, as pointed out above, the layout of a modern train with vestibules at the doorways means that each extra set of doors reduces the number of seats.
In my opinion, the best design of train for use on an intensive metro service was London Transport's C stock, with four double doors per side. I'm very pleased that the Crossrail stock will have three doors. It means that every passenger will be that bit nearer a doorway which should speed up boarding and alighting. (Actually, at most stations the order of events is alighting and boarding.)
I know that the DfT specified detailed requirements for the Thameslink stock, but with only two doors per side I wonder whether, in practice, a crowded train will get delayed when a large number of passengers want to get off and a similar number want to get on. It only needs a problem on one coach out of twelve to delay the whole train.
For trains that will be used on intensive commuter services my preference is strongly for three doorways per side. It will mean at least eight fewer seats and maybe twelve, but if most passengers are going to have to stand anyway it makes sense to design the train so that people can get on and off as quickly as possible.
