Which of the following is NOT a key component of a train's air brake system?

Answer: D — Train signal system

A train's air brake system relies on various components to function effectively in order to ensure the safety and reliability of train operations. An air compressor generates the compressed air necessary to operate the brakes. The reservoir stores this compressed air, providing a ready supply for braking when required. The brake pipe is essential as it distributes the compressed air throughout the train to activate the brakes. The train signal system, while crucial for overall train operations and safety, does not form part of the air brake system. It primarily serves to communicate information about track conditions, train movements, and operational signals, rather than facilitating the physical operation of the braking mechanism. Understanding the distinct roles of these components helps clarify the functions critical to train safety and efficiency.