Damage to the overhead wires at Hayes & Harlington is likely to keep the Great Western Main Line closed between Ealing Broadway and Reading until around midday, according to Network Rail.
Engineers went to the scene early yesterday, after the incident apparently occurred at about 06.30.
All four lines are closed at the scene, and the main train service from London Paddington is the eastbound Elizabeth Line to Abbey Wood, although a limited shuttle has started running between Paddington and Ealing Broadway about every 30 minutes.
It is reported that about 2km of wire was damaged, and repairs are continuing.
The blockage caused problems yesterday for thousands of people travelling to London from the west of England and south Wales to the state funeral of her Majesty Queen Elizabeth II. They had to change at Reading for trains to London Waterloo, and that remains the principal travel advice this morning.
However, Oxford and Banbury passengers can also use Chiltern Railways to and from London Marylebone.
The cause of the damage has not yet been revealed.