When may a switch be left open for a following train?

A switch may be left open for a following train only if it is in the charge of a crew member of that train. This means that there has to be someone present and accountable for the switch position, ensuring that safety protocols are followed. The crew member in charge can then monitor the switch's status and make decisions based on their immediate circumstances, enhancing safety and operational efficiency.

An open switch presents a risk if there is no oversight; therefore, having a crew member responsible for its status mitigates that risk. Other options, while they might seem plausible, do not meet the requirement of having proper oversight or accountability that the crew member brings. This principle ensures that safety is the primary focus in railroad operations.