Southern and First Capital Connect trains are once again serving London Bridge Station after a partial closure lasting more than a week.
"The huge amount of work we have completed in the last nine days is an essential part of the jigsaw that will eventually give passengers new more spacious trains running at tube like frequency through central London, and connecting them to more destinations that ever before," said Simon Blanchflower, Network Rail's major programme director.
"We try our best to minimise the impact on passengers and do most of the work behind the scenes, but there are big pieces of work that can only be completed when the railway is clear of trains and people.
"We do this at the quietist times, such as the August bank holiday, but we understand the impact our work has on the daily lives of passengers we'd like to thank them all for bearing with us whilst we got it done."
While the part of the station used by Southern trains has been closed, two new platforms (12 and 13) have been brought into use. The old platforms 10 and 11 have been demolished to make way for the next phase of rebuilding of the station.
New track and new signalling equipment has been installed on the approach to London Bridge, which Network Rail claims will make journeys more reliable and reducing frustrating waits outside the station.
At Christmas the station will see another part closure to allow the next set of rebuilt platforms to be brought into use.
In January 2015 work to start the rebuild of the second half of the station begins with longer-term changes to Southeastern services.
Rail minister Claire Perry MP said: "Thameslink is a key feature of the record levels of investment we are making in our railways as part of our long-term economic plan.
"Last week I visited London Bridge and saw for myself how hard everyone in the Thameslink family has worked to reach this major milestone. I know this work has disrupted passengers' journeys and I would like to thank them all for their patience while this important work was under way.
"Once the Thameslink project is completed it will transform travel across London and the South East for hundreds of thousands of passengers who travel into the capital every day."
On Sunday 14 September the train services currently run by First Capital Connect will be transferred to the new Govia Thameslink Railway franchise.