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.

'''IMPORTANT: When a detector rail is placed besides a regular rail, the game will crash! Until this gets fixed you can work around this by placing the detector rail before any nearby regular rails. This doesn't work when trying to place the detector in between two parallel tracks.'''

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

Short video showing how to craft new rails and properties

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, this 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.