How will a driver know if the park brake has failed to release?

The correct choice highlights that if the park brake has failed to release, the park brake (PB) rotary switch will remain illuminated. This illumination serves as a direct indicator to the driver that the park brake is still engaged and has not functioned as expected. The operational design of train systems usually incorporates safety indicators like illuminated switches to alert the driver of any malfunction.

If the PB rotary switch remains illuminated, it implies that the release mechanism has not completed properly, giving the driver clear feedback that action is needed before proceeding. This prevents the driver from operating the train under unsafe conditions.

In contrast, other choices would not provide a direct indication of a park brake failure. The train rolling back may occur in situations of incline, but that does not definitively indicate a park brake issue. Likewise, a continuous horn sound is not typically associated with the failure of the park brake but may relate to other operational alerts. A significant drop in brake pressure could indicate a different problem entirely, such as a leak or failure in the brake system, not specifically a failure to release the park brake.