Two new platforms have been opened at London Bridge in a "major milestone" for the Thameslink programme.
The work involved the creation of the platforms, the laying of new track and the installation of a state-of-the-art signalling system.
More than 1,200 engineers worked around the clock to complete the project, while train operators supported the works by running amended timetables and running alternative services.
Rail minister Claire Perry said: "Thameslink will transform travel across London and the south-east for hundreds of thousands of passengers who travel into the capital every day."
The Thameslink programme, which is due to be completed in 2018, will give passengers more choice and convenience when travelling through London and the south east.
More reliable journeys and better connections will be among the benefits of the scheme, which will bring new trains running every two or three minutes to the heart of London.