The other day I happened to notice something strange in the New York City subway: while waiting for my train and letting a couple of others pass, I caught sight of something peculiar—each time, a window opens in the middle of the train car and the conductor points his finger at something. I looked closer and there it was: a black-and-white wooden sign bolted to the ceiling.

I looked it up and found out there's actually a rule: the conductor has to clearly point to this sign before opening the doors. It's kind of an indicator to make sure nobody steps out into the tunnel. They started doing this after 1996, when doors would open a few times while one or two train cars were still in the tunnel. I wonder if they actually check whether employees bother to follow the procedure?

The black-and-white sign is at every station (sometimes more than one, since trains can vary in length).

According to Wikipedia, this practice is widespread in Japan (though I didn't see anything like it when I was there; guess I'll have to go back).