What paper is required for a train to enter the main track from a siding?

The requirement for a train to enter the main track from a siding is the Clearance Card Form A. This document acts as a formal authorization, ensuring that the train is properly cleared to access the main track safely. The Clearance Card contains vital information about the train's movement, including track conditions and any potential conflicts with other train movements. It plays a crucial role in maintaining operational safety and preventing accidents, as it notifies personnel that the track is clear for the train to proceed.

In contrast, the other options do not fulfill the specific requirement for entering the main track. The timetable provides information about train schedules and routes but does not serve as authorization for movement. Form L is generally used for reporting the status of wayside signals or other operational issues rather than for clearance to enter the main track. Blue signals indicate work in progress or maintenance activity on the track, acting as a warning for nearby personnel but not serving as a form of clearance for trains. Thus, Clearance Card Form A is the correct and necessary document for this procedure.