When is it permissible for a train to accelerate back to track speed after encountering a permanent low speed crossing sign?

The permissible action of a train accelerating back to track speed after encountering a permanent low speed crossing sign is contingent upon safety considerations, particularly ensuring that the entire train has cleared the crossing zone. In this context, the leading motive power unit clearing the crossing signifies that the front of the train has passed the point where the restriction applies, indicating that it is safe to resume normal operations.

This decision is rooted in the need to ensure that all components of the train are no longer within the zone of reduced speed, which is critical for the safety of the vehicle as well as any other road or rail users that may be affected by the train's movements.

Other options may not fully account for the necessity of the entire train’s clearance before acceleration can occur. For instance, waiting for the last car to clear might seem reasonable, but it does not provide the clarity that the leading unit's clearance offers regarding the immediate safety of the track ahead. Additionally, waiting for a signal or an increase in speed limit could create unnecessary delays or confusion in operations, as these conditions may not accurately reflect the safety status of the track after the crossing. Thus, the requirement is focused on the leading unit as a clear indicator to resume regular speed safely.