London Paddington's mainline rail services were disrupted after 90m of signalling cable was stolen from the station in west London.
The theft left all signals on the four lines in and out of Paddington red, rendering all lines bar one out of action.
Only one train an hour was running in either direction between Paddington and Oxford, Penzance and Glastonbury.
Network Rail said the cables have been reconnected and engineers are testing them.
The services are expected to return to normal today.
CrossCountry Trains, Chiltern Railways, South West Trains and London Underground have been accepting tickets on alternative rail and tube routes.
A 48-hour strike by tube maintenance workers is also expected to have hit commuters in London.