What must happen when a train is stopping on a spring switch?

When a train is stopping on a spring switch, controlling the slack is essential for safety and effective operation. When trains come to a stop on spring switches, the design of these switches allows for movement of the rails to accommodate a slight bending or shifting, which is critical to ensure the switch remains operational and properly aligned.

If a train were to release all slack, there could be unnecessary tension or jerking as the train comes to a standstill, which could potentially lead to misalignment of the tracks or the switch itself. The goal is to maintain an even tension on the train cars to prevent them from impacting each other violently or displacing the switch components.

Moreover, moving forward slowly is important, but it is more related to controlling how the train approaches the switch rather than the act of stopping on the switch itself. While sounding a horn is a good practice for alerting personnel, it does not directly relate to operations concerning the condition of the switch at the moment of stopping. Hence, controlling the slack ensures the train is safely positioned on the spring switch without compromising the integrity of the switch or track.