The Detector - Empty is a block from the Railcraft mod. This block is meant to output a Redstone signal based on the type of Minecart that passes in front of it. The Detector - Empty will output a Redstone signal if the Minecart in front of it can be ridden, but has no entities in it. They can be placed under or beside Minecart Tracks, and will output a signal to the side that has a small square. When a Redstone signal is active this square will turn red.