What safety device prevents excessive speeds during emergency braking?

The dynamic brake is designed to convert the kinetic energy of the train into electrical energy, which can then be dissipated as heat. This mechanism allows for smoother deceleration, thereby helping to maintain control of the train's speed, especially in emergency situations. When the dynamic brake is engaged, it assists in reducing speed more effectively than relying solely on friction brakes. This capability is crucial for preventing excessive speeds that might occur during emergency braking scenarios, ensuring a safer stopping process.

The other options do not fulfill this specific role. The throttle controls the power output of the locomotive and does not aid in braking. The speedometer measures speed but does not intervene or control speed in emergencies. The emergency brake valve activates the emergency braking system, but it is the dynamic brake that specifically aids in managing speed during such braking events.