What is the primary function of a wheel slip indicator?

The primary function of a wheel slip indicator is to alert engineers of potential traction issues in real time. This tool plays a crucial role in maintaining the safety and efficiency of train operations. When a train experiences wheel slip, it indicates that the wheels are losing traction, which can lead to reduced control, increased stopping distances, and potential damage to the rail and wheels.

By providing immediate feedback about the status of wheel slip, the indicator allows engineers to make informed decisions, such as adjusting the throttle or braking systems to counteract the slipping. This real-time monitoring helps prevent accidents and ensures the train operates smoothly under varying conditions.

The other choices do not align with the primary purpose of the wheel slip indicator. Measuring wheel diameter is critical for maintenance and assessment of track compatibility but is unrelated to traction monitoring. Indicating tire pressure is typically associated with road vehicles rather than trains, while monitoring fuel efficiency is a separate aspect of locomotive performance that does not directly involve traction assessment.