What does a red lantern on the rear of a car on a lay-up track indicate?

A red lantern on the rear of a car on a lay-up track serves as a critical safety signal. It indicates that workers are on or beneath the car, thereby alerting anyone approaching that caution must be exercised. This is essential for preventing accidents, as other train operators or maintenance personnel will recognize that they should not operate any trains in that area until it's clear and safe to do so.

The other choices do not accurately reflect the specific purpose of a red lantern. For instance, a car that is ready for service would typically not display a red lantern, as that would indicate the car is in working order and safe for movement. Similarly, a signal indicating that the track is blocked ahead would usually involve different signage or markers, and a car marked as out of fuel would not specifically be identified by a red lantern either. Thus, the use of a red lantern is exclusively tied to worker safety around the train.