Which statement best describes how CBTC determines safe spacing between trains?

• A. It uses moving blocks and continuous communication rather than fixed physical blocks.
• B. It uses no communication and relies on train timing only.
• C. It uses fixed track blocks with no moving parts.
• D. It relies solely on driver input for spacing.

Answer: (A)

CBTC determines safe spacing by using moving blocks and continuous communication rather than fixed physical blocks. In a moving-block system, the boundary that defines the safe distance behind the lead train isn’t tied to a fixed section of track. Instead, the boundary shifts with the lead train’s position, speed, and braking capability, and the trailing train’s onboard computer continuously computes the minimum stopping distance and sets a safe speed to maintain that dynamic gap.

Two-way communication between trains and the control system provides real-time data on position, velocity, braking status, and occupancy. This lets the following train adjust its speed and door-to-door spacing on the fly, achieving tighter, safer headways than traditional fixed-block signaling allows. The emphasis is on real-time data and dynamic envelopes rather than static track sections, which is why relying on fixed blocks or on driver input alone wouldn’t describe how CBTC ensures safe spacing.