The head of Network Rail says the new concourse at one of the capital's busiest railways will not solve commuter problems for the next 50 year.
Two thirds of the development at London Bridge station was unveiled on Monday which will enable passengers to access all platforms from one level for the first time.
But Mark Carne, who was speaking at the opening of the new concourse, said he was not confident that it would meet the needs of passengers in the long term.
Mr Carne said that there had been a "staggering" growth in passenger numbers on services that pass through London Bridge of 5% to 6% every year.
Over the bank holiday weekend the project's construction site was moved away from the Southern and future Thameslink platforms to focus on the north of the station, used by Southeastern.
Trains into Charing Cross have now begun calling at London Bridge 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.
Mr Carne said he was "very excited" about the changes that had been made and apologised to passengers who had been affected by the delays.