Mourners travelling to the Queen's funeral procession could miss the event due to blocked rail lines between Reading and London Paddington.
It means disruption for people travelling via Berkshire and Heathrow Airport.
Great Western Railway (GWR), Heathrow Express and the Elizabeth line are all affected.
National Rail Enquiries said "major disruption" was likely until the end of the day, with potential cancellations and delays of up to 90 minutes.
The disruption is also affecting people travelling from Paddington to areas in the West - including Windsor, where the Queen's coffin will be taken after the state funeral service.
Officials said earlier that all public viewing areas on the London procession route are already full.
Passengers on a GWR train bound for Paddington were told over the tannoy earlier: "My sincerest apologies for the delays on such an important day for the country."
One traveller tweeted: "I'm stuck on the train for two hours. Got up at 4am to get to Windsor early for the viewing. Absolutely horrible service.
"Barely even caught this train because nobody at Paddington knew which one it was. Sure didn't expect this dismal service."
Another passenger tweeted that he had been "evacuated" from a train just outside Paddington and "unceremoniously dumped on the side of the road",
"Now in an Uber with four other passengers to Windsor," posted @rgshanks.
The rail problems began at around 6.30am and were caused by damage to overhead cables at Hayes and Harlington, towards the edge of west London.
Lines between Reading and Newbury were also closed after a person was hit by a train, and GWR trains were diverted.
People wanting to get to London from Reading were being advised to use South Western rail into Waterloo.
Travellers on GWR could use their ticket on Chiltern Railways, London Underground or Southern trains.
People on the Elizabeth line could use their ticket on the Tube, London buses, First Berkshire between Slough and Reading and on South Western Railway between Waterloo and Reading.
Drivers had to plan their journeys carefully and were being urged to stay at home if possible.
Many roads in central London were closed as well as major routes in west London around the A4 and A30 - which the funeral cortege will use to get to Windsor.
Some road closures will last into the evening and bus routes may also be diverted or cut short.