What must occur when the train approaches a public crossing at grade?

When a train approaches a public crossing at grade, it is essential to activate safety measures to ensure the protection of both the train crew and the public. Continuous ringing of the bell serves as an auditory warning to pedestrians and motorists about the approaching train. This sound alerts individuals in the vicinity to exercise caution and prepare to stop or move clear of the tracks.

As for the other options, while dimming the headlight may be appropriate in certain circumstances or for specific types of trains, it is not a universal requirement at crossings. Ensuring that all passengers are seated is relevant for safely managing passenger movement but does not directly contribute to the safety protocols at a crossing. Similarly, while reducing speed is a prudent safety measure, the primary obligation at crossings specifically emphasizes the continuous ringing of the bell as a crucial warning system.