Two thirds of a new concourse opens at London Bridge railway station on Monday as part of a major redevelopment.
Once completed in January 2018 it will be bigger than the pitch at Wembley Stadium and the largest of any station in Britain.
Passengers will be able to access all platforms from one concourse at London Bridge for the first time.
Over the bank holiday weekend the project's construction site is being moved away from the Southern and future Thameslink platforms to focus on the north of the station, used by Southeastern.
Trains into Charing Cross will begin calling at London Bridge on Monday for the first time since January 2015, but Cannon Street trains stopped serving the station on Friday and will not resume until January 2018.
The south London station has suffered incidents of severe overcrowding since work began four years ago.
Reconfiguring the complex track layout around the station has exacerbated the impact of any delays.
Network Rail (NR), which owns and operates Britain's railway tracks, signals and busiest stations, including London Bridge, said the improvements will allow up to 24 Thameslink trains an hour to run through the capital - equivalent to one every two to three minutes - compared with just eight previously.
There will also be more connections to Gatwick and Luton airports, and beyond to Peterborough and Cambridge.
NR chief executive, Mark Carne, claimed the redevelopment will "transform north-south travel through London".
He said: "We are rebuilding Britain's fourth busiest station - the tracks, the platforms and the infrastructure which enables trains to run - all the while keeping the station open and doing our best to keep passengers moving.
"I am extremely sorry that while doing this work there have been times when passengers have suffered frustrating delays.
"I know that the promises about tomorrow are of little consolation when performance isn't good enough today.
"I am pleased that, finally, some of the benefits of this project will be much more visible."