Activator Rail

The Activator Rail is a block that is used to activate TNT minecarts. When given a redstone signal, they turn on and will active any cart that goes over them. When off, they act as normal rails. They pass power in the same way as powered rails do. Activator rails do not accelerate minecarts, or affect any besides the TNT minecart.

Trivia
In 13w02b, you can't craft this item. (Not confirmed in single player.)