Extra trains have been laid on from the west of England to cope with the large numbers of people expected to travel to London for the Queen's funeral.
Train operators have warned all services will be "extremely busy".
The extended services will run through until Tuesday morning to help people travelling from further afield.
However, operators have warned travellers not to attempt to go to London for the funeral and then to Windsor for the committal service.
There has been high demand for the extra services put on by Great Western Railway (GWR), since Wednesday, for people wanting to see the Queen lying-in-state at Westminster Hall.
Even more trains have been added for people travelling to the state funeral on Monday, including overnight.
These will run from Monday night to early hours of Tuesday morning from London Paddington, through Reading, Swindon, Chippenham, Bath and on to Bristol Temple Meads.
As it is a bank holiday, normal ticket restrictions have been lifted.
GWR said: "It will not be possible to view both the funeral in London and travel to Windsor, as Windsor and Eton Riverside and Windsor and Eton Central stations will be especially busy during this time and may become full.
"You should plan to view one or the other and allow plenty of time to travel to either location."
GWR spokesman Paul Gentleman added: "This is a huge logistical operation to make sure it all goes off as smoothly as possible and that people get to pay their respects in the way they want to to Her Majesty the Queen.
"That's why we've put extra services in on some key routes, and some overnight trains as well."
He advised passengers to check the GWR online journey planner for times, as well as travel updates on Twitter.
"We expect those trains to be very, very busy, but we do recognise that this is an event like no other," he added.
South Western Railway, which runs services to London Waterloo, said it would have a full weekday timetable on Monday with a "limited number of additional services where possible".
Motorists who plan to drive in and out of the capital are advised to allow plenty of extra time and stagger their journeys where possible.
National Highways has also warned motorways and major roads around London will be very busy.
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