Complex sentence: Gatwick Express, formerly a separate franchise but now operated by Southern, runs from platforms 13 and 14 at Victoria and is a shuttle service between London Victoria and Gatwick Airport every 15 minutes.
Split into simpler sentences: Gatwick Express, formerly a separate franchise but now operated by Southern, runs from platforms 13 and 14. It is a shuttle service between London Victoria and Gatwick Airport every 15 minutes.