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 TNT minecart or the hopper minecart.

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.

If there's a hopper in the minecart above, a powered activator rail will disable the hopper.

Snapshot 13w04a: If a minecart with TNT passes by an activator rail on an adjacent block, the minecart with TNT will explode instantly, destroying the activator rail and the adjacent tracks.

Bugs
Some times when you log into minecraft, the activator rail will not have a name. This also happens with the texture pack button