Unless brakes fail en route, how many cars in a train must have operative brakes?

Answer: B — All cars: 100%

The requirement that all cars in a train must have operative brakes ensures the overall safety and stability of train operations. Having 100% of the cars equipped with functioning brakes allows for more effective control over the train's speed and enhances the ability to stop safely in emergencies. This is especially important given the significant weight and momentum of freight trains, where even a single car without brakes can compromise the entire train's stopping capability and increase the risk of accidents or derailments. In practice, adhering to this standard minimizes the potential for dangerous situations, such as uncontrolled movements, and contributes to better compliance with regulatory and safety standards in rail operations. It promotes an environment where the risk of incidents due to brake failures is significantly reduced, reinforcing the importance of operational brakes in maintaining train safety.