Activator Rail

The Activator Rail is a block that is used to activate TNT minecarts or Minecarts with Hopper. When given a redstone signal, they turn on and will activate 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 Minecart with TNT or the Minecart with Hopper. They also do not occur naturally and must be crafted.

Behavior
When a minecart with a TNT block is above a powered activator rail, the fuse will be set off causing the minecart to explode after a certain period of time. The faster the minecart goes over the activator rail, the more powerful the explosion is.

If there's a hopper in the minecart above, a powered activator rail will disable the hopper. If a disabled hopper minecart is sent over an unpowered activator rail the cart will be re-enabled. Also, breaking a hopper minecart will cause it to be enabled the next time you place it.