When is it permissible to leave equipment on a main track?

Leaving equipment on a main track is governed by strict safety regulations to ensure the smooth operation of train services and to prevent accidents. The correct answer pertains to the specific scenario of fouling a switch.

When equipment is allowed to foul a switch and that switch is lined for the other track, it means that the configuration of the switch directs traffic away from the equipment that is on the track. This allows for safe passage of trains on the designated route without interference from the equipment. The fouled position is less critical in this scenario because the switch is actively set for the alternate track, preventing potential collisions or disruptions.

In contrast, if equipment were left on the main track while trains are scheduled (as suggested by other choices), or if the track alignment does not provide safety measures, it could lead to very dangerous situations. Other options, such as being lined for the corresponding track or the presence of crew members, do not adequately address the critical aspect of ensuring that train movements remain unaffected by fouled clearances. Therefore, option C provides the most contextually appropriate scenario under which it is permissible to leave equipment on a main track.