What is required to display at the rear of every train when operating on main track?

To operate safely on the main track, it is essential for every train to display a marker at the rear. This marker serves as a warning and identification tool for other trains and railway personnel, indicating the end of the train. It is crucial for ensuring safe operations and allowing other trains to be aware of the train's length and positioning on the track, particularly in low visibility conditions or at night.

The use of a marker at the rear enhances overall safety by preventing collisions and enabling the effective management of train movements. In contrast, other options listed, such as a light signal, blue flag, or warning sign, do not fulfill the specific requirement for marking the end of a train on the main track, making the marker the correct choice for this scenario.