Why is a mandatory stop required in rainy weather?

A mandatory stop in rainy weather is primarily required because yard tracks may become slippery. When it rains, the moisture can create slippery conditions on the tracks, which can significantly impact the train's traction and braking capabilities. This can pose a safety risk, as it becomes harder for the train to stop or control its speed effectively. Making a mandatory stop allows operators to assess the conditions and ensure that the train can navigate the track safely, which is crucial for both passenger safety and the maintenance of equipment.

The other options do not address the primary safety concerns associated with operating a train in wet conditions. Checking for fallen debris is important but not directly tied to why a mandatory stop is specifically necessary in rain. Frequent service may be hindered by weather conditions rather than enhanced, and ensuring all passengers have boarded is a standard operational procedure, unrelated to weather conditions specifically.