What would indicate a problem if there were a lack of compressed air in a train?

The inability to start the train is directly related to the function and operation of various systems that rely on compressed air. In many trains, compressed air is essential for starting the engine, as it powers the pneumatic systems necessary for ignition and operation of key components. When there is a lack of compressed air, it signifies a critical failure in supplying necessary pressure, which prevents the start-up sequence from being completed. Thus, the absence of compressed air indicates a problem that would prevent the train from starting, making this the most accurate answer among the given options.

While the other options involve systems that may also rely on compressed air but do not specifically tie the problem to starting the train directly. For instance, communication systems and speedometers can have separate power and operational sources, and door locking mechanisms may not exclusively depend on compressed air. Therefore, the context of starting the train makes the first choice the most valid indication of a problem.