When may train speed be increased after passing a yellow-red flag?

The correct answer is based on the established safety protocols for train operations when encountering a yellow-red flag. A yellow-red flag typically indicates that there is a reduced speed requirement for trains in that area due to potential hazards or conditions that have not been fully cleared.

Increasing the speed only after the leading wheels of the train are 4 miles beyond the yellow-red flag provides a clear buffer and ensures that the train is completely past the potentially hazardous area before normal operations can resume. This distance is a precautionary measure to enhance safety, allowing both the crew and the infrastructure time to respond to any unforeseen issues.

The other options may suggest alternatives for increasing speed but do not adhere to the specific safety protocols that govern train operations around yellow-red flags. Therefore, option C is the most appropriate choice for ensuring safety in a railway environment where caution is necessary.