Thousands of train commuters in the Thames Valley suffered severe disruptions to their journeys on Thursday because of a signal failure.
The problem occurred between Reading and Twyford, causing cancellation and delays to London Paddington services.
First Great Western cancelled several trains and others were delayed for more than an hour after a cable was cut.
The company's managing director later took the unusual step of publishing an open letter of apology to customers.
'Lessons learnt'
In the letter published on the company's website, Alison Forster said: "I am writing to apologise on behalf of ourselves and Network Rail for the severe disruption to this morning's services into London Paddington.
"We are already discussing with Network Rail how the fault occurred and what they can do to minimise future failures of this nature.
"We will ensure that lessons are learnt and acted upon.
"Customers had a truly awful service this morning and we will, of course, pay compensation."
The loss of signalling in the area was blamed on a cable that was cut during overnight engineering work.
Network Rail said officials had worked through the night to repair the cables.
The problem was fixed shortly before lunchtime on Thursday, but disruptions to the London train service remained.