Mainline rail services in and out of London Paddington station have been severely disrupted after 300ft (90m) of signalling cable was stolen.
Only one train per hour is running in each direction between Paddington and Oxford, Penzance via Bristol, Swansea and Castle Cary, for Glastonbury.
Thousands of passengers were hit when the cable was taken from West Drayton, west London, on Wednesday morning.
It is hoped services will be back to normal on Thursday morning.
Tickets are being accepted on alternative rail and Tube routes.
CrossCountry Trains, Chiltern Railways and South West Trains are accepting them, as well as London Underground.
Wednesday's tickets can be used instead on Thursday, First Great Western added.
The theft had a knock-on effect for Wednesday evening's rush-hour and also affected plans for those travelling to screenings of England's World Cup match against Slovenia.
Engineers at scene
The vandalism meant all signals on the four lines in and out of Paddington turned red and services had to be "drastically" cut, a Network Rail spokesman said.
As a result only one of the four lines could be used fully, with one completely out of action and two partially disrupted.
The cables have now been reconnected and engineers will work through the night to test them, Network Rail said.
"Our aim is to restore services for the morning peak tomorrow," the spokesman added.
"If you don't need to make your journey tonight, it'd be best not to.
"Otherwise prepare for crowded conditions and leave extra time for your journey."
Commuters in London also faced potential disruption on Wednesday evening because of a 48-hour strike by Tube maintenance workers from 1900 BST.