Minecraft wiki entry for Redstone_components, associated text: An activator rail affects certain minecarts passing over it. The effects vary with the type of minecart activated:
A minecart with command block executes its command every 2 redstone ticks (5 times per second).
A minecart with hopper is deactivated by an activated activator rail (it stops sucking up items in its path, or transferring items to containers as it passes them), and re-activated by an unactivated activator rail.
A minecart with TNT is ignited by an active activator rail.
Regular minecarts with an entity riding it (mob or player) eject that entity if the activator rail is active.
Other minecarts are not affected by an activator rail.