Complex sentence: Passengers who cannot use steps and are boarding or alighting from trains to Cork and Limerick via Limerick Junction must cross the tracks at ground level, which is only possible when the train is clear of the tracks.
Split into simpler sentences: Passengers who cannot use steps and are boarding or alighting from trains to Cork and Limerick via Limerick Junction must cross the tracks at ground level. This is only possible when the train is clear of the tracks.