Name the device used to measure the speed of a locomotive.

The device used to measure the speed of a locomotive is the speedometer. This instrument provides real-time information on how fast the train is traveling, which is crucial for safe operation and adherence to speed limits. Speedometers are specifically designed for this purpose, allowing engineers to monitor and control speed effectively to ensure safety and compliance with regulations.

An accelerometer, on the other hand, measures the rate of acceleration rather than speed, providing data on how quickly a train is speeding up or slowing down but not its current speed. An altimeter measures altitude or changes in elevation, which is not relevant to speed measurements. A chronometer is a device used for timekeeping and could be used in calculating speed indirectly, but it does not directly measure speed itself. Thus, the speedometer is essential for directly gauging locomotive speed.