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.

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. Some most notable would be activating a note block to indicate if someone/something is in a minecart. It could also be used to create a train station like feature. For example, A cart could be looping using the new Powered Rail while going over 1 Detector Rail. When the detector rail detects a person is in the cart, It could activate redstone, triggering a normal rail to change state, thus making the cart break the loop and travel onto a seperate track. However, this is currently not the case, as unoccupied minecarts will still trigger a detector rail.