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.