What speed may a train with failed cab signals proceed at once authorized by Rule 556?

The correct answer is the speed limited by fixed signals only. When a train experiences a failure in its cab signals, Rule 556 of the NORAC provides guidance on how to proceed safely. In this scenario, trains are allowed to operate at speeds dictated solely by the fixed signals that are present along their route.

This approach ensures that safety is maintained, as the crew will rely on visual indications from signals rather than automatic cab signal systems that may have failed. Therefore, the train must observe the indications given by the fixed signals, which can vary depending on the area, the train's location, and the conditions governing the operation at that specific point on the rail network.

The other speed options provided—50MPH, 40MPH, and 59MPH—do not accurately reflect the protocol established under Rule 556 for a situation where cab signal failures are involved. Instead, knowing that the maximum permissible speed is determined by fixed signals illustrates the emphasis on safety and adherence to the signaling system in place.