When a train unexpectedly stops while passing over a switch, the Tower Operator should FIRST?

In a situation where a train unexpectedly stops while passing over a switch, the immediate priority is to ensure the safety of the train and its crew. Issuing a Call-On signal on any Home Signal the train is bridging allows the train to proceed with caution and helps to manage the situation effectively. This action is crucial because it informs the train crew that they can proceed slowly and safely, even though the normal signal indications are not clear due to the train's unexpected stop.

Following this, the operator can then address other necessary operational procedures, such as checking for track obstructions or notifying the train dispatcher. However, the first step in this scenario is to provide a means for the train to safely continue past the switch to avoid any further complications, such as blocking the main line or creating a hazard for other trains. Hence, initiating a Call-On signal is the appropriate response in this scenario to maintain safety and allow for a controlled continuation of operations.