What might happen when air pressure in the brake system is insufficient?

When air pressure in the brake system is insufficient, the brakes may fail to engage properly. Air brakes rely on a specific level of air pressure to function effectively. If this pressure is too low, it can lead to incomplete braking action, meaning the train may not be able to slow down or stop as intended. This situation poses significant safety risks, as proper braking is essential for controlling the movement of the train, particularly in emergencies or when descending steep grades.

The other possible outcomes listed would not occur with insufficient air pressure. For example, a train cannot stop more quickly if the brakes are not engaging effectively, and train operation would not remain unchanged because the lack of proper braking directly affects the train's functionality. Furthermore, insufficient air pressure does not contribute to improved fuel efficiency; rather, it can lead to other complications in operations.