Thousands of commuters were delayed after scheduled engineering work overran, "trapping" 13 First Great Western "Turbo" trains in Oxfordshire.
Network Rail blamed equipment failure for the delay, which led to the Oxford and Didcot Parkway line being closed.
Services to London Paddington, Reading, Berkshire and Oxford were severely affected, said First Great Western.
Rail operator FGW said services could not move from about 0550 to 0750 BST. Network Rail apologised for the delays.
"Equipment failure in the early hours of this morning meant that we were unable to complete important engineering works to schedule," a spokesman said.
"A full investigation into this incident will take place over the coming days.
"We sincerely apologise for the inconvenience this has caused passengers."