Final answer:
The correct action when approaching an ungated, unlit railroad crossing with an oncoming train is to stop fully and wait for the train to pass before proceeding.
Step-by-step explanation:
If you are approaching a railroad crossing that does not have gates or lights and you see an oncoming train, the safest action to take is to stop and wait until the train passes. This ensures that there is no risk of a collision with the train. Trying to speed up to beat the train, slowing down but still proceeding, or turning around to find an alternate route are not safe actions in this scenario, as they could lead to dangerous situations.