As Joe points out, the cord on the newest cars isn’t an actual emergency brake. A traditional emergency brake cord physically dumps the air out of the brake system, forcing the train to stop. In the newer cars, a computer decides whether the train should stop (if part of the train is still in the station) or not (if it isn’t).

Do you open the emergency exit door on an airplane in any sort of emergency? Of course not – most emergencies don’t call for immediate exiting. So why would you activate an emergency brake if you don’t want to immediately stop the train?

The only semi-legitimate use I can really think of is if the doors close at the wrong time, separating a parent from a young child (could be a disobedient child, or a parent struggling with a stroller).

I realize that bureaucracies speak an Alice-in-Wonderland style of English with different meanings for words and everything, but the signs they put up for the general public should be in normal English. If you put up a sign for normal people saying “Emergency Brake”, people will pull it in, well, emergencies. That is what the sign says. If you don’t want people to pull the cord in emergencies, don’t put a sign up saying, “pull this cord in emergencies”.

Personally, I’ve only ridden the subway a few times, and have read the sign accompanying the emergency brake, and found it to be clear. But if someone never bothers to read the instructions? Tough to handle that scenario.