Complex sentence: Between London Bridge and the vicinity of Stoney Street, an extra pair of tracks will be on a new viaduct to the south of the existing one which will be used by trains to Charing Cross.
Split into simpler sentences: Between London Bridge and the vicinity of Stoney Street there will be an extra pair of tracks on a new viaduct to the south of the existing one. The southern pair of tracks will be used by trains to Charing Cross.