Detector Rail

Detector Rails are a type of rail that acts like a pressure plate, and the first rail switch.

When any kind of cart is on top of the Detector Rail, it will generate a redstone signal. The signal will be transmitted to blocks that are adjacent to the detector (including Powered Rail) and redstone placed under the rail, like pressure plates.

Crafting
Detector rails are crafted as normal rails, but with a pressure plate instead of a stick, and the addition of redstone.

Uses
Detector rails can be used for many things. One train station-like feature could be activating a Note Block to indicate that a minecart is approaching, or a note block in conjunction with a redstone torch to have a grade crossing signal. It is easiest to simply wire a series of 4-6 detector rails, one around every 20 blocks, to a note block and an inverter that powers a Redstone Torch. As the cart passes each detector rail, it will trigger the circuit for one flash of the torch and a chime, thus giving you 4-6 chimes before the cart reaches the crossing.

Detector rails can be used with a memory cell to power powered rails for long tracks.

Additionally, they can be used to create a one-way gate to a Powered Rail, with any carts coming the other way slowing down or stopping completely.

They also can be used to activate dispensers filled with arrows to create a minecart-friendly mob trap.

As a defense mechanism in multi-player or for your own challenge, you can make a rectangle using powered rails, regular rails, and detector rails. Then, make a box around it with glass (except the floor). At every detector rail have a redstone wire leading to a dispenser filled with arrows. When you begin the movement of the mine carts they'll pass over the detector, firing the arrows in a constant pattern until you run out of ammo. It is also good for speed killing by filling a hole with mobs and slaughtering the mobs inside; you can get a lot of resources this way.

They can be used for multi-layered piston doors with repeaters and redstone.