Complex sentence: Stations in the north London and some South London suburbs are served every 15 minutes, while stations in Central London are served every 15 minutes for each service, however stations on the Wimbledon Loop Line are served every 30 minutes.
Split into simpler sentences: Stations in the north London and some South London suburbs are served every 15 minutes, while stations in Central London are served every 15 minutes for each service. Stations on the Wimbledon Loop Line are served every 30 minutes.