A Victorian railway bridge in Reading is being rebuilt so trains can run over it at full speed again.
Network Rail said trains had been running more slowly over the Huntley and Palmers bridge over a former railway tunnel "as a precaution".
Known as "Biscuit Tunnel", because it was used to transport biscuits from the town's factory, it is now used as a pedestrian and cycle path.
The work will mean disruption to rail services over the weekend.
On Saturday buses will replace trains between Wokingham and Reading. On Sunday buses will replace services between Ascot and Reading, and Reading to Guildford.
On both days train services for Gatwick Airport will start from Guildford. CrossCountry services for Guildford will start and terminate at Reading.
Network Rail said the bridge on the Reading to Wokingham line was "now past its sell by date".
"In recent years trains on the Reading to Wokingham line have been forced to run at lower speeds over the bridge in order to keep pedestrians and cyclists safe."
The rail operator said a brick facade would be added to the tunnel entrance at a later date to retain "the look and feel of the original bridge".