The Infrared Detector (IRD) is an electronic device that uses infrared light to detect trains. They are ideal for giving the operator a visual indication when a train has reached a certain location on your layout that is obscured from view. When a train passes over the sensor an infrared beam is reflected off the bottom of your train causing the LED to be turned on. There is no tracks to cut or modifications to rolling stock, simply mount the detector at the required location underneath your track.