What is the immediate action required if a signal changes from a proceed indication abruptly?

When a signal changes from a proceed indication suddenly, the immediate action required is to stop immediately. This response is critical for maintaining safety on the railway. A sudden change in a signal indicates that there may be an obstruction on the tracks, a red signal ahead, or other hazardous conditions that necessitate an immediate halt to prevent a potential collision or derailment.

Stopping promptly allows the train crew to assess the situation and respond appropriately, whether that means communicating with dispatch, evaluating the signaling conditions, or ensuring the safety of personnel and equipment. This immediate stop action aligns with safety protocols designed to protect both the crew members and any trains operating on adjacent tracks, mitigating risks associated with unexpected signal changes.

Continuing at current speed would pose a significant risk, as would slowing down cautiously without first assessing the situation. Checking for signs may provide additional information, but the priority in a signal failure is to halt train operations until the safety of the track ahead can be confirmed.