If one light is covered and the other is green, it indicates that the track is clear and that traffic can proceed. A red light indicates that a vehicle or pedestrian must come to a compete stop before entering a railroad crossing.
There may be instances when both lights are lit at a railroad crossing. Green over red instructs the engineer of the train to slow down and to prepare to come to a stop at the next signal. Red over green tells the engineer to be ready to stop, but that the train will be able to proceed after coming to a stop.
Railroads use two different types of signals: permissive and absolute. Permissive signals allow the engineer to decide whether or not to go and absolute signals must be followed without any decision-making on the engineer's part. Absolute signals are most often found at railroad crossings, main track crossovers and drawbridges, according to Trains magazine.
A semaphore lighting system is a small tower with colored light pairs attached to it. This is the system to look for when approaching an area with railroad tracks near it.
