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 rail (including powered rails) and Redstone placed under the rail, much like how Pressure Plates work.
|x6 Detector Rail|
|Iron Ingot||None||Iron Ingot|
|Iron Ingot||Stone Pressure Plate||Iron Ingot|
|Iron Ingot||Redstone||Iron Ingot|
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 one 4-6 chimes before the cart reaches the crossing. 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. They also can be used to activate dispensers to create a minecart-friendly mob trap.
- If a detector rail is on a slope with a powered rail adjacent to it on flat ground, it will not activate the powered rail when a minecart runs over it.