I've been in a class that was canceled before. They put you on a short list and you'll be at the top of the list for call back when they do start hiring. They'll just call out of the blue a year later and ask if you're still interested. Happened for me. The most important thing is, get started as soon as you can where ever there is a class, accept it. Senority is EVERYTHING, and what matters is your senority date. Later you can transfer around.
There is one exception, up on the Northern Lines there are lots of places that are islands. No system wide senority. So in those places you lose you existing senority when you go in and you lose it if you come out. Systemwide, versus local senority, it is like art. If it's currently helping you it's wonderful and the guys don't complain, but if its hurting you, then guys complain. So there is no sure answer. One's as good as the next.