Almost 80,000 passengers from Thames Valley and the West Country who use the station daily will be affected by re-scheduled services.
BR staff carrying out routine maintenence work at Dolphin Junction, near Slough, Berkshire, severed three signal cables yesterday morning causing electrical relays to blow.
A BR spokesman said: 'It could take two days to repair the faults because we have to restore the main power supply to the signals as well as repair trackside cables, some of which are 30 years old.'
The disruption will hit InterCity services from South Wales, Bristol and the West Country. Berkshire commuter routes affected will include those from Oxford and the Thames Valley, Reading, Newbury and Hungerford. Many trains on the line from Oxford, Didcot and Newbury to Paddington will either be cancelled or terminate at Reading. Some services from Reading will be routed via Waterloo.