London Victoria station
This split is generally held to, as the track layout does not allow much swapping, with only a small number of connecting flyovers between the main lines in the Battersea area, and a single track connection just outside the station. As the Brighton side is the busier of the two, disruption on that line sometimes results in some of its suburban services using the eastern side.  This is often true of the Gatwick Express, which travels along the Brighton Main Line, as it will often divert over Chatham side tracks during engineering works in order to maintain service levels.