Briefly describe how a DC track circuit detects a train in a block.

Briefly describe how a DC track circuit detects a train in a block.

• A. Optical sensors detect train.
• B. A high DC current is injected; occupancy detected.
• C. A low DC current is injected into the rails; when a train is present, the axle and wheel set disturb the circuit and occupancy is detected.
• D. Temperature sensors on rails detect heat.

Answer: (C)

DC track circuits feed a small direct current into the rails to define a block. In an empty block, the current flows through the rails normally and the detector monitors a steady electrical condition. When a train enters, the axle and wheelset provide a conductive bridge between the two rails, changing the impedance in that section of track. This change in current or voltage is picked up by the track detector, which marks the block as occupied. The use of a low current helps keep heat down and makes the occupancy change easy to sense. Optical or thermal sensing isn’t how this DC block detection works, since the system relies on the electrical interaction created by the train’s metal wheels bridging the rails.

DC track circuits feed a small direct current into the rails to define a block. In an empty block, the current flows through the rails normally and the detector monitors a steady electrical condition. When a train enters, the axle and wheelset provide a conductive bridge between the two rails, changing the impedance in that section of track. This change in current or voltage is picked up by the track detector, which marks the block as occupied. The use of a low current helps keep heat down and makes the occupancy change easy to sense. Optical or thermal sensing isn’t how this DC block detection works, since the system relies on the electrical interaction created by the train’s metal wheels bridging the rails.