An extra three trains a day now run between Blackfriars and Kent's county town on weekdays, complementing the existing commuter service between Blackfriars and Ashford via Maidstone East.
The new trains run every three hours during the day on Monday to Friday only, with all trains stopping at Elephant & Castle.
The new Southeastern service is a partial replacement for the planned Thameslink service which was due to link Maidstone with Cambridge. Whilst the Cambridge trains were intended to run via London Bridge, the new service take the other route to Blackfriars via Elephant & Castle.
Steve White, Southeastern's managing director, said: "Our focus is on winning back our customers, reversing the shift from public transport to cars that took place during the pandemic, and so we're really pleased to offer these additional services to our timetable between Maidstone East and the City of London.
"Rail is the greener way to travel and we want to make our service an easy and compelling choice for people to make. From this week there are an extra six trains per day on this route, Monday to Friday, with a travel time of just over 60 minutes."
However the introduction of the new Southeastern service has led to a temporary paring back of the Thameslink service to Sutton with extra gaps in the timetable when the Maidstone train is occupying a platform at Blackfriars.
This temporary cutback won't be necessary once Thameslink restores its full timetable with all Sutton trains continuing through central London to St Pancras International and beyond.
• Elephant & Castle is due to gain further new rail links in May this year when a half-hourly rush hour Thameslink service to Welwyn Garden City via Finsbury Park is introduced.