Detector Rail

Detector Rails are blocks that were added in 1.5. They are planned to act like pressure plates but will be 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.

You can view a video of booster/powered rails and detector rails here

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