Who must be notified when a car's brake equipment fails en route?

Answer: B — Relieving Crew and Dispatcher

When a car's brake equipment fails during transit, it is essential to notify the relieving crew and dispatcher. This is critical for ensuring the safety of the train operation and for implementing the necessary procedures to address the brake failure. The relieving crew needs to be informed so they can prepare for any additional actions required once they take over operating duties. Simultaneously, the dispatcher must be made aware of the situation to facilitate communication with other trains on the route and to help coordinate any necessary response, such as arranging for mechanical assistance or modifying schedules. Effective communication among these personnel is vital for maintaining operational safety and efficiency in response to equipment failures. While other choices include notifying the train engineer or all crew members, the focus on the relieving crew and dispatcher specifically captures the operational chain of responsibility for dealing with failures in brake equipment while on the move.