What happens to the signal under CBTC when there is a failure?

When a failure occurs under a Communication-Based Train Control (CBTC) system, the signal is forced to red. This is a safety measure designed to prevent any potential accidents that could occur if a train were to proceed through an area where the system is not functioning correctly. By maintaining the signal at red, the system ensures that trains are halted and can approach the failure point with caution.

CBTC systems rely heavily on real-time data communication between the trains and the trackside systems. When a failure is detected, the priority is to ensure the safety of train operations, thus defaulting to the most restrictive signal aspect – in this case, a red signal. This proactive approach minimizes the risk of crashes by preventing trains from entering potentially unsafe sections of the track until the situation can be assessed and resolved.

In contrast, other options suggest different responses that do not align with the operational protocols of CBTC systems, where safety is paramount, and a red signal is a standard safety response in the case of failure.