A "very limited" train service will run through the night on some routes as officials brace for "unprecedented" crowds to descend on London to see the Queen's coffin.
Journey planners showing exactly which routes will be updated later this week, the Rail Delivery Group said, as hundreds of thousands of people come to see the late sovereign lying in state in Westminster Hall.
The 14th Century banqueting hall will be open from 5pm on Wednesday to 6.30am on Monday, the day of the state funeral.
But fresh reports today suggest more than half the 750,000 mourners expected to queue in the capital could be disappointed, as the queue will be closed early if it is too busy.
According to The Times, officials estimate a 17 to 35-hour waiting time for the route, expected to stretch five miles from Southwark Park and along the South Bank of the Thames to Lambeth Bridge before doubling back.
Last night the Rail Delivery Group warned London "will be exceptionally busy" and planning journeys ahead of time "will be essential". No10 said the capital's commuters may consider working from home.
There could be "short-notice temporary Tube station closures" and passengers arriving from outside London should consider walking to their final destination in the capital, the RDG said.
The east-west Elizabeth line - named for the late monarch - will run a special service with 12 trains per hour on the central section from Paddington to Abbey Wood on Sunday 18 September.
Travellers are asked to avoid Green Park Tube station, next to Buckingham Palace, and buses will be diverted due to road closures.
A Rail Delivery Group spokesperson said: "The rail industry is doing all it can to ensure people can travel to pay their respects during the period of mourning.
"Where possible we will be running some extra services and customers should check journey planners for the most up to date information.
"A very limited service will be operating through the night on some routes, people should check journey planners for details."
Vanessa Nathakumaran, 56, was first to join the London queue yesterday despite organisers asking people not to wait in the capital until tomorrow. People are ordered not to camp.
Full details of the queue line - which will have portable toilets, food stalls and wristbands to allow people to rest - will be published at 10pm tonight.
Ms Nathakumaran, from Harrow, told the Mirror: “I was taking a walk, I asked them what they were setting up for, I felt a strange sense of feeling overwhelmed, but I knew I must do my duty.
“It is very emotional to know that she passed on, she’s done a great service."
An industry source told the Telegraph there was a "dilemma" about laying on extra trains as it could give a "green light" for people to make travel plans to London.
They added: "If anything, [the government] want to suppress demand rather than encourage it.”
Crowds of thousands will turn out today as Queen Elizabeth II ’s coffin continues on its final journey to London.
The oak casket will land at RAF Northolt after flying 400 miles from Edinburgh Airport.
And fans will line the streets for the 15-mile journey to Buckingham Palace, where Her Late Majesty will rest for the evening as staff pay their respects.
At 2.22pm on Wednesday, the coffin will be taken on a gun carriage of The King’s Troop Royal Horse Artillery to the Palace of Westminster.
Accompanied by the sound of minute guns fired from Hyde Park by The King’s Troop Royal Horse Artillery, the cortege will make a trip past London landmarks.
It will travel via Queen’s Gardens, The Mall, Horse Guards and Horse Guards Arch, Whitehall, Parliament Street, Parliament Square and New Palace Yard.
Downing Street refused to rule out some people being turned away from the queue to the lying-in-state if it gets too busy.
No10 also did not deny reports the waiting time has been estimated at 17 to 35 hours, saying it “will be very long”.
But the PM’s spokesman said the government was not asking mourners to stay away from the capital, adding: “We do want as many people as possible to be able to come to the lying in state.
“This is a significant period for the United Kingdom... What we’re doing is being up-front with what we expect to be significant numbers of people.”
The PM’s spokesman said he was unaware of any “upper cap” to the number of people who can pass through Westminster Hall - after reports only 350,000 will be able to file through in four and a half days, leaving many disappointed.
While he said decisions on access to the queue will have to be made on the ground, “we don’t know exactly how many people will attend, albeit we expect it to be very large numbers.
“I don’t think there’s necessarily going to be a cutoff point - our overarching aim is to make sure as many people as possible can get through.”
There will be 1,500 soldiers on standby to help stewards, and there will be a “significant operation” for police to keep mourners safe.
Queuers will be able to buy refreshments through the night, No10 said, and there will be portable toilets and “lots of stewards”.
It’s understood there will be two separate queues - a main queue and a shorter accessible queue for those who have disabilities or mobility issues.
Stewards will be able to use their judgement to decide who qualifies for the shorter queue - focusing on people who are “not physically able to wait that long”.
It was not immediately clear how this will work in practice, when the starts of the queues are miles apart.
