Train services from Oxfordshire to London Paddington have been severely disrupted after a theft of copper cabling at Tilehurst.
First Great Western says alterations, cancellations, delays, and a much reduced service can be expected.
Network Rail says services could remain disrupted for the remainder of Friday.
It is the second copper theft from the line this week. A theft on Wednesday caused disruption for hundreds of passengers in the afternoon.
In 2005 there were almost 1,000 incidents of theft of copper signalling cable from warehouses or from along rail lines, a rise of 50% on the previous year.