Abstract: A detector circuit for a model railroad employs two pairs of magnetically-operated relays positioned on either side of a grade crossing along a set of tracks. In addition a single magnetically-operated relay is located at the grade crossing and is given an orientation orthogonal to that of the two pairs of relays. A locomotive of the model railroad is equipped with a first magnet for operating the pairs of relays and at least one car has a second magnet for operating the relay at the grade crossing. A control circuit is connected to the relays such that crossing gates at the grade crossing are lowered, a bell is sounded, and lights are flashed whenever the locomotive magnet passes over the pairs of relays traveling toward the grade crossing. The control circuit also raises the crossing gate and deactivates the bell and flashing lights whenever the car magnet moves over the relay at the grade crossing or the locomotive magnet moves over one of the pairs of relays while traveling away from the grade crossing.