The £10m road bridge was launched across the 10 operational rail lines that form the main lines into London Bridge at Tennison Road, Norwood Junction, in south London.
All the while, trains continue to pass beneath.
Network Rail and its contractor Graham Construction (under a £6.7m contract) are midway through a project to replace the original bridge with a bigger, better structure, spanning 55 metres. The new bridge will have wider footways and carriageway.
The original Tennison Road Bridge has spanned the main lines between London Bridge and East Croydon since 1922. The new bridge was launched into position using a hydraulic strand jack system.
Network Rail's scheme project manager, Shane O'Connor, said: "A combination of technology and engineering expertise meant we could keep trains and passengers moving while we the bridge was launched across the tracks.
“This is the first time we’ve worked in this way – in the past the only way would have involved a complete closure of the railway, which would have caused huge disruption. It's a great step forward for passengers and it's also a big leap forward for Croydon, as it will soon have a much improved bridge with extended paths and a wider road."
Graham Construction project manager Rory McFadden said: "Launching a 565-tonne bridge over 10 operational rail lines is quite a feat of engineering but the innovative methods used allowed for a controlled and safe installation. That said, it took a lot of preparation in advance of the actual launch.”
Tennison Road Bridge is scheduled to open for public use in spring 2015.