What must be ensured regarding the brakes of a train for safety compliance?

For safety compliance, it is essential to ensure that at least a specific percentage of the train's cars have operative brakes. This requirement is critical because having functional brakes on a significant portion of the train ensures that it can be effectively controlled during operation, particularly in emergency situations or when needing to stop quickly. The braking system is a vital safety feature that helps prevent derailments and collisions by allowing for adequate stopping power across the entire length of the train.

While other options touch upon aspects of operational safety, they do not align with the key requirement that directly addresses the necessity of having a certain percentage of operational brakes across the train. Manual operation of all brakes would not be practical or safe for long trains, and ensuring maintenance of brakes for an entire journey is certainly important, but it is the percentage of operational brakes that is specifically stipulated for compliance and active safety measures. Focusing on distributing braking power effectively among train cars is fundamental in train handling and safety standards.