At what speed must a locomotive have an operative speed indicator?

The correct answer, which states that a locomotive must have an operative speed indicator above 20 MPH, is grounded in safety protocols and operational efficiency. Speed indicators are critical instruments that provide crucial information about the locomotive's speed to the engineer, allowing them to operate the locomotive safely within the limits set for various conditions and environments.

Having a speed indicator functional at speeds exceeding 20 MPH is essential because this speed threshold is where the dynamics and forces acting on the train change significantly, making it necessary for the engineer to have accurate data to make informed decisions. Operating at higher speeds without this information could lead to safety hazards, such as overshooting signals or excessive wear on the rails.

Additionally, regulations often stipulate equipment functionality for safety and compliance purposes, ensuring that trains operate within prescribed limits to protect both crew and cargo.