Train delays caused by thieves who stole copper wire from a signal point have hit "thousands of passengers".
A reduced service is running between Reading, Oxford, Swindon and London after damage was caused near Tilehurst.
Trains that are running are facing 60-minute delays with a "significantly reduced" number of services in operation, especially around Reading.
Passengers have been warned that the disruption is likely to last for most of Friday.
In a separate incident, a signalling issue means there is a replacement bus service between Aldershot and Ascot.
Network Rail appealed to anyone with information about the theft to contact police.
A spokesperson said: "Many thousands of passengers are being unnecessarily delayed today thanks to an act of mindless vandalism.
"Not only does cable theft cause massive disruption and delays nationwide, it also deprives passengers of vital investment in their railway.
"Last year we paid out almost £8m in compensation to train operators for disruption caused by thieves - money that otherwise would have been spent improving services."