What device helps prevent over-speeding of a locomotive?

A speedometer is a vital instrument that measures and indicates the speed of the locomotive. It provides the engineer with the necessary information to monitor the train's speed continuously. By keeping an eye on the speedometer, the engineer can make informed decisions regarding throttle application and speed reduction to maintain safe operation within designated speed limits. This proactive monitoring helps prevent over-speeding, which can lead to dangerous situations such as derailments or collisions.

Throttle control, while crucial for regulating speed, does not specifically provide feedback on how fast the locomotive is going, thus lacking the direct measure that aids in preventing over-speeding. The brake system is essential for slowing down or stopping the train but does not actively prevent over-speeding; rather, it is a reactive measure. The coupling mechanism functions to connect the locomotive and cars but is unrelated to speed regulation. These tools and systems work together, but the speedometer is the specific device that aids in consistently monitoring and preventing excessive speed.