Every morning when I hop off the subway to go to work, I notice an MTA employee standing at the end of the platform, waving a flashlight over the heads of the people getting on/off the train. I can’t quite figure out what these guys are signaling. (It has to be a signal of some kind, right?) I don’t know how many stations this happens at–it doesn’t happen where I get on, it does where I get off, and I seem to remember seeing it elsewhere at some point.
My not-particularly-educated guess is that it has something to do with alerting the operator that the last few cars have finished boarding and it’s safe to go, but that’d still leave the operator guessing as to what was going on in the middle of the train.
The guy who drives the train (the train operator) is in the front, but the guy who controls the doors and signals when it’s safe to move (the conductor) is in the middle.
At some stations, especially if the track is curved, the view from the middle cab along the platform may be obscured, or it may just be so crowded at rush hour that the conductor can’t easily see the length of the train. In these cases there will be spotters at the end of the platform to signal the conductor that it’s safe to close the door and nobody’s going to be dragged by the train. At other stations, video monitors in the middle of the platform provide the same service.