Final answer:
The maximum safe speed at which a light engine can operate without any risks or safety concerns.
Step-by-step explanation:
The question asks about the speed limit for a light engine operating on a single light-rail commuter train with no cars on the main line. In this case, the speed at which the train should not be exceeded is the top speed or maximum safe speed at which the train can operate without any risks or safety concerns. This speed will depend on various factors like the train's design, track conditions, and operational guidelines.