What should a train do if the previous indication is unknown?

When the previous indication is unknown, the appropriate action is to proceed at restricted speed until the leading wheels have passed the next governing signal or the end of the block system. This response is rooted in safety protocols designed to manage uncertainty on the tracks. Restricted speed allows the train to react to any unforeseen obstacles or signals that may be encountered ahead, thereby minimizing the risk of accidents.

By proceeding at a controlled speed, the train crew can ensure they maintain safe operations while still advancing towards the next signal and remaining vigilant for any changes in track or signal conditions. This approach promotes a cautious method of navigation in situations where clarity regarding the prior signal is lacking and helps maintain safe distances from potential hazards or other trains on the same line.

In contrast, alternatives such as stopping immediately would unnecessarily halt operations without assessing further situational information, while continuing at normal or full speed could lead to dangerous situations if the unknown indication poses any risks. Therefore, proceeding at restricted speed serves as a balanced and prudent tactic in managing the operational uncertainty caused by an unknown train indication.