What type of station stop only requires the Engineer to stop for visible passengers?

The correct choice is based on the definition and operational procedures associated with a Flag Stop, also known as an F Stop. At a Flag Stop, the train is only required to stop if there are visible passengers waiting to board or alight. This type of stop is typically used in less busy areas or on specific routes where not all stations warrant regular service.

This operational procedure allows for flexibility in scheduling and can help reduce travel time on routes with limited ridership. The requirement for visible passengers ensures that resources are allocated efficiently, avoiding unnecessary stops when there are no passengers. This approach is less stringent than a Regular Stop, where the train must stop regardless of passenger presence, and more specific than Timed Stops, which are predetermined stops regardless of passenger visibility. Express Stops are used primarily for high-speed services, where the train does not make stops at all except at designated stations.