Minecraft wiki entry for Activator_Rail, associated text: ejects mobs (including players) from minecarts as if a player had used the sneak control to exit the minecart (by default, one block to the right of the direction the minecart is moving and up to two blocks up, unless blocked)
repeatedly activates command block minecarts — a command block minecart executes its command every four game ticks until the command block minecart is no longer on the activator rail
disables hopper minecarts — a disabled hopper minecart cannot pick up items it travels through until it is enabled by an inactive activator rail (or until it is broken and re-placed)
primes TNT minecarts — primed TNT minecarts explode after four seconds with an explosive power proportional to their speed over the activator rail.