You have picked a subject that is good for books, week long professional seminars, and extensive studies. Anything here can hardly more than scratch the subject.
Appropriate for conditions is the key starting point.
A high volume system with closely spaced stations needs cars with multiple doors per side, at least three, or even four, minimal seating, usually a bench style on each side, etc. You do want windows that go high enough that the standees can see out, as you must be able to see where you are. That also means you need to have multiple large easily seen signs with the station name on them in every station. If you have a multi-lingual population, the signs should be also, particularly if not all styles of wirting used are in the Roman alphabet. The above description would apply to places like New York, Tokyo, Hong Kong, etc.
A lower volume system that has wider spaced stations can have more seating and fewer doors per side. San Francisco BART cars have only two doors per side and transverse seating. Again, you still need the windows and the station signs, and BART is very poor on the signage score.
Appropriate for climate is also important. Ground temperature also goes higher as you get closer to the equator. Therefore, the ground is not a good "heat sink" in tropical climates, meaning the system ventilation must be able to move much larger volumes of air, otherwise the air in the tunnel can get so hot that the train air conditioning fails.
In Hong Kong, Singapore, Taipei, and probably else where, but these I know, there are no doors between cars. Instead there are open gangways, with vestibule bellows, of course, so that there is easy passage the full length of the train. This helps even out passenger loading, but I regard it as a smoke/fire hazard, in that any fire anywhere in the train results not in a car full of smoke, but a train full of smoke.
