Door etiquette on trains could be an entirely separate category. But it shouldn't be this hard, as everything is common courtesy. Standing on the platform and the doors open? Let those inside get off first before boarding. Don't try to smash in between those who are disembarking.
If you're approaching a car and the doors start to close, do not rush forward and push them open. Train doors are not like elevator doors – they won't open back when they sense an obstruction so your foot, briefcase or arm might just get stuck. In worst-case scenarios, the train operator can't get the door to reopen and close properly after an obstruction so all the passengers have to disembark and wait for the next carrier [source: Hedgepeth]. Imagine how popular you'll be, then. Just keep it simple and wait for the next train if you're running late.
Once inside, take a seat, grab a pole or find a spot to stand -- but not directly in front of the doors. The only exception is if you're going to get off at the very next stop. Getting off in two stops? Back away, especially if you have a suitcase or big bag with you. If you don't, you'll be blocking those trying to get on and off at the next stop. If the train is so crowded that you're forced to stand next to the door anyway, get off at the next stop to allow others to disembark and then reboard.