Rail passengers face significant disruption over Christmas as a number of lines are closed for engineering work.
Network Rail (NR), which manages Britain's rail infrastructure, is carrying out up to 200 improvement projects during the festive period.
Work on some schemes will begin on Christmas Eve - which falls on a Saturday - to give engineers and maintenance teams four days to carry out upgrades before the next working day.
London, Manchester and Cardiff are among the cities affected.
Passengers are advised to plan their journeys in advance using the National Rail Enquiries website.
Phil Hufton, NR's managing director for England and Wales, said engineering work is carried out throughout the year but larger upgrades usually take place over bank holidays as up to 50% fewer passengers travel on those days.
He went on: "Despite our essential upgrades, the vast majority of the rail network - more than 95% - remains unaffected and will be open for business as usual.
"This vital investment will make the railway more reliable for years to come, while improving journeys for thousands of passengers.
"There's never a good time to impact on journeys and I'd like to thank passengers in advance for their patience."
No trains will operate to or from London Paddington between December 24 and December 29 because of work to build the new Crossrail rail link.
That means the Heathrow Express service between Heathrow Airport and Paddington will be suspended for six days and Great Western Railway trains will terminate at Ealing Broadway.
Major upgrades of signalling, track and overhead lines for Crossrail's eastern section mean bus replacements and diversions will be in place for services to and from London Liverpool Street from December 24 to January 2.
Work on Manchester's Ordsall Chord scheme, to create a link between the city centre's main train stations, means rail replacement buses and diversions will be operational for Oxford Road services from December 18 to December 28.
There will be no late-night trains between Cardiff Central, Bridgend, Newport and the Valleys on Christmas Eve due to significant work around the Welsh capital to bring new signalling and tracks into use. There will be further disruption from December 27 until January 3.
Work at New Cross, south London as part of the Thameslink programme means no Southeastern trains will operate to London Bridge and Charing Cross from December 24-28, apart from the Hastings services which will be diverted to London Bridge.