Tennison Road bridge, near Norwood Junction, will close this week in preparation for demolition to allow it to be replaced with a bigger and better structure.
The bridge, which has spanned the main lines from London Bridge and Victoria to East Croydon since 1922, has to be replaced as it has reached the end of its life.
The main span of the new structure, which will be hydraulically pushed into place next Christmas, will be 3.5m wider to allow for wider footways, and is due to open for public use in spring next year.
Demolition of the bridge will be a gradual process and there will be some changes to train services at various weekends during the autumn to allow for the safe removal of the existing bridge.