What must not happen while a switch is being operated?

When operating a switch, it is crucial that equipment does not foul the switch. Fouling occurs when equipment is positioned such that it might interfere with the movement or setting of the switch, potentially causing a dangerous situation. For a switch to function safely and effectively, the area around it must be clear of obstructions, allowing for smooth operation and minimizing the risk of accidents or equipment damage.

While employees should exercise caution in the vicinity of a switch (which is addressed in one of the other choices), the primary concern during the operation of a switch is ensuring that nothing obstructs its movement. This is vital for both the safety of personnel and the proper functioning of the railway operations. If equipment fouls a switch, it could lead to improper alignment or failure of the switch, posing significant risks during train operations.