A permit is not necessary for bringing bicycles on NYC subways, but is required for passage aboard the Long Island Railroad (LIRR) and Metro North Railroad (MNR) systems.
Permits cost $5 and are good for life. In other words, as long as you hold onto the card, a one-time fee of five bucks gives you lifetime allowance to bring your bike aboard trains so long as it’s within MTA guidelines. Bikes are prohibited during peak hours, but if you’re departing the city in the morning and returning in the evening there shouldn’t be a problem.
Permits can be purchased on the spot at window 27 in Grand Central Terminal or other outlying staffed ticket offices. For more info, visit the MTA.