Under what condition are workers permitted to occupy a railway track?

The correct choice emphasizes the critical safety protocol that workers must follow before occupying a railway track. The requirement to have proper clearance ensures that workers verify the area is safe and that no trains are approaching. This is crucial when working near or on railway tracks, as trains can approach quickly and often silently.

While the other options present important aspects of safety training or supervision, they do not encompass the immediate and vital condition of ensuring that the track is clear of oncoming trains before any worker enters the area. Training on track safety prepares workers for these situations, and having a supervisor or designated maintenance times may provide additional layers of supervision and planning, but the primary concern must always be the protection against moving trains. Therefore, the best practice is to only occupy a railway track after confirming that it is safe to do so.