What must a train ascertain before leaving a station when the Rule 251 is in effect?

When Rule 251 is in effect, a train must ascertain the arrival of superior opposing regular trains before departing a station. This rule is applicable in single-track areas where trains may encounter each other while traveling in opposing directions. The purpose of this requirement is to ensure safe operations by confirming that no higher-priority trains are approaching or occupying the track section that the departing train intends to use.

Knowing the status of opposing trains is crucial for maintaining both safety and efficiency on the rail system. It allows the crew to avoid potential conflicts and coordinate movements accordingly, thereby preventing accidents and delays. This focus on monitoring opposing train movements is a fundamental aspect of operating procedures in areas governed by Rule 251.

The other options, while relevant to general train operations, do not directly pertain to the specific requirements set forth by Rule 251 regarding train departures.