The detector rail is a type of rail block that acts similar to how a pressure plate works. It puts out a redstone current when any kind of minecart is on top of it. The signal will be transmitted to blocks that are adjacent to the rail and redstone placed under the rail, much like how pressure plates work.
A detector rail (via screenshots)
Detector rails can be found in the chest minecrafts of abandoned mineshafts but they can also be crafted with six iron ingots, a stone pressure plate, and redstone dust. You can pick up naturally generated detector rails with the help of a pickaxe.
Crafting a detector rail (via screenshot)
Detector rails are used for many things. Since they output a redstone signal whenever a minecart goes over it, it can be used as a sort of signal to alert players of an approaching minecart. Since they are rails they do act like regular rails allowing minecart to run on them. The minecarts lose a little momentum when moving on them.
When a minecart is on top of a detector rail it will produce a redstone signal with a power level of 15. When the minecart is moved the detector will cease emitting the redstone signal. If the minecart on the detector rail is a minecart with chest or minecart with hopper, an adjacent redstone comparator facing away from the detector rail outputs a power level proportional to the container’s fullness, possibly power level 0.
A detector rail activating some powered rails (via screenshot)
Detector rails are blocks that have the ability to output a redstone signal to power redstone circuits or just to provide simple signals just as note blocks for alerting players. They can also be used to measure how full are chests or hoppers in minecarts.
A. You will need six iron ingots, redstone dust, and a stone pressure plate.
A. Detector rails can be placed in front of powered rails to power them, but only when a minecart rides over the detector rail.
A. Detector rails act as pressure plates, which can be used to create signals and other functions when minecarts pass over them.