Under what condition may hy-rail vehicles move through the spring side of a spring frog?

Hy-rail vehicles are special vehicles that can operate on both highways and rail tracks, and their movement through a spring frog—a type of track switch mechanism—requires care to prevent derailment. The correct condition under which hy-rail vehicles may move through the spring side of a spring frog is to have another employee guard against derailment.

In this scenario, having another employee present serves as a safety measure. They can assist by providing guidance and alerts, ensuring that the hy-rail vehicle navigates the frog safely by monitoring the conditions of the track and the vehicle's movement. This proactive approach reduces the risk of accidents and ensures that the vehicle can proceed without the driver's sole oversight.

The other choices do not prioritize safety or do not comply with proper protocol. For instance, stopping before traveling and proceeding without checking does not allow for proper assessment of conditions, which could lead to unsafe movements. Proceeding slowly without stopping may still pose a risk if the conditions are not carefully monitored, while making a running start to clear the frog is inherently dangerous and increases the likelihood of derailment. Therefore, having another employee guard against derailment is the safest and most responsible option to take when navigating through a spring frog.