How must trains operate within the limits of a suspension in an ABS failure?

The correct answer indicates that passenger trains must not exceed 59 MPH, while freight trains must not exceed 49 MPH within the limits of a suspension in an Automatic Block Signal (ABS) failure. This approach is essential for maintaining safety in situations where the signaling system is not functioning properly.

By establishing speed limits specific to the type of train, these guidelines help manage the risk associated with reduced visibility or operational restrictions during an ABS failure. Passenger trains typically have a higher priority for passenger safety and are therefore subject to a slightly higher speed limit compared to freight trains, which can be slower due to their heavier loads and need for longer stopping distances.

Setting these specific speed limits helps prevent derailments and accidents that can occur if trains were allowed to operate at unrestricted speeds. It also ensures that train crews can appropriately respond to signals and any potential hazards that may arise on the tracks during such failures.

The context of the other options clarifies their impracticality or danger, as allowing unlimited speed or using discretion would significantly raise the risk of accidents, and only allowing freight trains to exceed the speed limits would not adequately address the safety concerns for passenger trains.