Detector Rail

Detector Rails are blocks that were added in Beta Version 1.5. They act like pressure plates but are a type of rail, allowing minecarts to move over them.

When any kind of minecart 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, meaning it behaves exactly like a pressure plate.

As of Beta 1.5, when a detector rail is placed beside a regular rail or another detector rail, Minecraft crashes due to an infinite loop. This can be worked around by placing the detector rail first, before placing regular rails nearby. This doesn't work when trying to place the detector in between two parallel tracks. During the crash moving minecarts may end up doubled. They also crash the game (only on mac?) when a minecart hits it.

Crafting
Detector rails are crafted as normal rails, but with a pressure plate instead of sticks, 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.

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 if not stopping completely.