What must occur before a train can increase speed from a cab signal indicator?

The correct answer highlights that for a train to safely increase speed based on a cab signal indicator, it must first have traveled its full length. This requirement ensures that the entire train has cleared any potentially conflicting signals and is in a position where increasing speed would not create risks, such as entering a block occupied by another train or a track segment with restrictions.

This rule emphasizes the importance of ensuring that the train is entirely within a clear and safe area before it accelerates. It protects against situations where part of the train might still be in a restricted area, which could lead to safety incidents.

In contrast, confirmation from the Block Operator would be more about communication and coordination with dispatch but is not specifically required for increasing speed based solely on cab signal indicators. Monitoring for other trains is important for general safety but does not pertain directly to the mechanics of speed increases tied to cab signal indicators. Having no visible stop signals is also a consideration, but this condition alone does not account for the safety of the train traveling its full length first, which is the primary focus of this safety protocol.