What is the speed limit if both PTC and Cabs fail in ABS territory with a 556 signal?

In the context of the NORAC Dispatcher Practice Exam, when both the Positive Train Control (PTC) and Cabs fail within Automatic Block Signals (ABS) territory, the standard speed limit is set at 40 mph when a 556 signal is displayed. The 556 signal indicates that train movement is permitted, but with specific restrictions due to the failures of these safety systems.

This speed limit serves as a precaution in situations where advanced control mechanisms have become inoperative, thus ensuring greater safety in operations. It is essential to adhere to this limit because it helps minimize the risk of accidents that could arise from higher speeds in potentially hazardous conditions where monitoring and control might be compromised.

Other options indicate higher speeds which don’t reflect the reduced safety protocols established for this scenario. Compliance with the 40 mph limit is vital to maintain adequate control and response time under degraded circumstances.