Which condition is less than optimal for a train's braking system?

A train's braking system relies on a properly pressurized air supply to function effectively. When the system is undercharged, it means that there is not enough air pressure present to apply the brakes efficiently. This condition can lead to a delayed or insufficient braking response, making it challenging to stop the train safely or to control its speed effectively. An undercharged system increases the risk of accidents because the train may not be able to respond appropriately to emergency situations or when gradual stopping is necessary.

In contrast, a fully charged system allows for effective and immediate brake application, while a gradually charged system can still maintain functionality as it builds up pressure over time. An overcharged system can create its own issues, but it does not compromise the train's ability to stop as significantly as an undercharged system does. Hence, an undercharged braking system is considered the least optimal condition for safe train operation.