Activator Rail

An activator rail is a type of rail that can "activate" certain minecarts. When powered with redstone, activator rails will trigger specific effects on minecarts with TNT, a hopper, or a command block that pass over. When turned off, they act as normal rails. Activator rails do not generate anywhere, and must be crafted to obtain them in survival mode.

Crafting
Activator rails are crafted similarly to normal rails, with the exception of additional sticks and a redstone torch placed on the middle.

Effects
If a minecart with TNT is on top of a powered activator rail, the fuse will be set off causing the minecart to explode after a certain amount of time. The faster it passes over the activator rail, the more powerful the explosion.

If a minecart with hopper is on top of a powered activator rail, the minecart will turn off, disabling its ability to pick up/drop items and blocks. If a disabled hopper minecart is sent over an non-powered 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.

If a minecart with command block is on top of a powered activator rail, the command block will run its command.

If a player or entity (like a mob) is in a minecart while the activator rail is powered, it will be ejected as if the sneak key is pressed. This functionality was added in snapshot 14w05a

Powering
Power can be transmitted to the rail from any of the six adjacent positions (above, below, or any side) in the same way redstone is powered.

Activator rails will propagate power to each other if they are adjacent and part of the same track, for up to 9 blocks from the power source (1 being powered directly which is propagated to 8 adjacent rails). They will also receive power from any adjacent detector rails (when a cart passes over it), even if they are not part of the same track (which follows from the rules above).

Trivia

 * The activator rail uses the same texture as the powered rail, but without the gold line.