New trains run as part of £7 billion Thameslink programme
Rail passengers are beginning to experience the improved Thameslink network.
Preview services are running for the first time between Cambridge and Brighton, and Peterborough and Horsham.
Extra capacity for 40,000 more passengers will be created when the introduction of the new timetable begins in May.
The £7 billion project by public sector body Network Rail is set to transform north-south travel through London with new tunnels between St Pancras and King's Cross.
London Bridge station has been rebuilt and the track and signalling equipment in surrounding areas has been upgraded.
Nick Brown, chief operating officer of GTR, which runs Thameslink, said: "This is a landmark moment and shows that GTR is delivering on its promise to give passengers a new, high-intensity service to a greatly expanded network of stations."
Rail Minister Jo Johnson said: "Passengers on the expanded Thameslink route are now starting to see the benefits of almost 10 years of hard work.
"The ambitious £7 billion Thameslink programme - sponsored by this Government - is delivering extensive infrastructure enhancements, new trains and a new timetable to tackle one of the busiest and most congested parts of the rail network."
In October the Department for Transport (DfT) decided to delay the full introduction of new Thameslink
services from December 2018 to December 2019.
This reduces the risk of disruption but means passengers in some locations will not receive the full benefits of the programme for up to a year later than scheduled.
The Public Accounts Committee found that it took the DfT and Network Rail too long to start planning how the new railway will operate.
Thameslink services will step up to 18 trains per hour in May and 20 per hour in December, with the full increase to 24 not reached until December 2019.
It is not the first time an increase in services on the route has been delayed.
A modernisation scheme named Thameslink 2000 was first discussed in 1991 but did not begin until 2006 due to privatisation of the railways and complex planning inquiries.