Actually, yes. It's allowed at all times as far as I know. Or maybe it's not allowed during rush hour. The limitation on it is convenience. You have to lift your bike over the turnstile or ask a station attendant to let you in a special door. Then you have to carry the bike up and down stairs. Then you have to hold it on a crowded platform and take up a lot of space on a crowded train car. So not many people takes bikes on the subways often. But it can be done. In fact, my wife and I did a round trip on the subway with our bikes on Sunday, as we rode the Tour De Bronx, and we didn't want to add to all the riding on the tour.