Under what circumstance must a train provide head end protection?

Head end protection is critical in scenarios where there might be potential hazards or obstructions in the train's path. When a train is in manual block operation, particularly with a disabled train in proximity, it's essential to ensure that the safety of both the train and crew is maintained. The head end of the train, where the engineer and other crew members are located, needs to have a clear understanding of the situation to prevent any accidents or collisions.

In this context, head end protection serves to provide safety measures that alert the crew to the risk posed by the disabled train, enabling them to take appropriate actions. This could include notifying signalmen or communicating any necessary adjustments to operations to ensure safe passage.

The other options may not require head end protection in the same way. Signaling to another train does involve communication but is generally managed through established protocols and signals. Entering a yard may pose less risk, as yards are typically designed for lower speeds and have safety measures in place. Exceeding speed limits is a serious operational violation but does not specifically necessitate head end protection in a hazardous context like the manual block scenario does.