If I pay my $2.50 fare on the subway can I transfer to another subway within 2 hours?

As long as you don't leave the system (meaning, if you pass through a turnstile to exit), you can ride as many subway lines as you like. For instance, you can take the (R) from 86th Street to 59th Street, transfer to the (N), then go to Atlantic Avenue and transfer to the (2), and then go to 96th Street and transfer to the (1) train. Since you didn't leave the system, it won't cost you anything. But once you leave the system, you have to pay to re-enter. The only exception is when you're going from 59th & Lexington to 63rd & Lexington in Manhattan. There have actually been people who tried to break a world record for riding all the subway lines within the shortest amount of time.