Which system automatically enforces speed limits, movement authorities, and enforced stops to prevent collisions and overspeed derailments?

• A. ATC.
• B. PTC.
• C. Block signaling.
• D. Signal aspect.

Answer: (B)

Positive Train Control (PTC) is designed to automatically enforce speed limits, movement authorities, and required stops to prevent collisions and overspeed derailments. It combines onboard equipment with wayside systems to continuously monitor a train’s position and speed and to apply the brakes or stop the train when safety parameters are violated or when a movement is not authorized. This automated enforcement is what distinguishes PTC from other signaling concepts.

Block signaling mainly communicates track occupancy and movement permissions to crews and does not actively control train speed or perform automatic stopping. Signal aspect refers to the visual indication shown to the operator and does not enforce actions by itself. ATC is a broader term for automatic train control, but the system described—explicitly enforcing speeds and authorities with automatic stopping—is the PTC implementation.