In CBTC, what replaces fixed track blocks to determine safe spacing?

• A. Moving blocks replace fixed blocks to determine safe spacing.
• B. A manual control system.
• C. A physical barrier system.
• D. Fixed blocks with enhanced signals.

Answer: (B)

In CBTC, the idea is to replace static fixed track blocks with moving blocks. The system continuously communicates between trains and wayside equipment to know each train’s exact position and speed. With this real-time data, it calculates a safe following distance (the moving block) for each train and issues movement authority that adapts as conditions change. This lets trains operate closer together when braking and acceleration conditions allow, increasing capacity and efficiency compared with rigid, fixed-block sections.

Manual control isn’t what CBTC uses; the technology enables automatic or semi-automatic control through real-time data and braking/acceleration models, rather than relying on a human operator to manage spacing. Fixed blocks are a legacy approach that CBTC supersedes by defining safe spacing based on actual train states rather than predefined block boundaries.