[fiblye]

I think trains/subways aren't as much of a concern for most people. On a bus, it's not uncommon to end up with a stinky or fairly creepy person stuck next to you with nothing you can do. Trains make it easier to get up and walk away, even moving to another car if you want to.

[afuchs]

In New York City, it is illegal to move between subway cars, by using the end doors.

[watter]

I haven't spent much time in NYC but I had never seen people go through the doors as passengers until then. It seemed to be frequent/common too. I've since seen this once in another city and it was pretty clear that it was out-of-the-ordinary when it happened.

[fiblye]

Here in Japan it's perfectly alright, so it varies from place to place. I know the one time I went to New York I saw a guy passing through into other cars, but he didn't seem to be the type to care about the law.