As far as I can make out there are basically six different types of automatic external doors on trains on Britain (plus one type no longer found AFAIK): Two-leaf folding doors (AFAIK only found on the Class 139 Parry People Movers used on the Stourbridge branch) Four-leaf folding doors (on Class 142 Pacers when built, but I don't think any still have them now) Twin-leaf inward pivoting doors (AFAIK on all Pacers (Class 142/143/144) that are still in service) Conventional sliding doors (e.g. Class 156, 317, 319, 321, 455, 800/801/802, London Underground trains built in the 1980s or earlier) Externally hung sliding doors (most if not all London Underground trains built since the 1990s) Sliding plug doors (e.g. Class 170) Swing plug doors (e.g. Class 158, 159, 165, 166) What would you say are the main advantages and disadvantages of each type? I suppose conventional sliding doors are technologically the simplest, and therefore least liable to fail, but they have to open and close into a pocket, meaning that some seats do not have a window unless that space is used for something else such as a luggage stack. With sliding plug or swing plug doors you don't have that problem, but if they get stuck open then the train is out of gauge.