If a train's cab signal changes to a more favorable indication, when can the speed be increased?

The correct answer, indicating that speed can be increased after the train has run its length, is based on safety protocols established to ensure that the train is fully under control and the crew can adequately observe the signal's changes without compromising safety.

When a cab signal changes to a more favorable indication, the train crew must ensure that all portions of the train have passed the previous signal indication. This rule is in place to prevent scenarios where a change in signal could lead to confusion or misjudgment if the crew were to respond immediately to the signal without confirming that they have safely passed the previous stop indicator. By running the length of the train first, it allows for a confirmation that all parts of the train are clear of any potential restrictions that could arise from the signal just passed.

Increasing speed immediately or upon seeing the signal could lead to dangerous situations if not every section of the train is clear of a potential stop or caution ahead. Likewise, waiting for permission from the Control Center could create unnecessary delays and may not align with operational practices, specifically if the signals indicate a clear progression through the route. Hence, the safest and most compliant practice is to wait until the entire length of the train has cleared the previous signal before increasing speed.