Rail firms have ramped up warnings to passengers travelling over the bank holiday weekend with major engineering work affecting several lines.
Virgin Trains issued an alert stating that its services will be "very busy" over the long weekend.
High demand is expected for east coast trains between London King's Cross and Scotland due to passengers seeking alternative routes to and from London because of the closure of London Euston on Saturday and Sunday, as well as Edinburgh's festivals coming to an end.
A Virgin spokesman said: "We expect our services on both routes to be very busy from Friday to Monday, and are advising customers to avoid travelling on our west coast route on Saturday or Sunday unless necessary.
"If customers choose to travel, we strongly recommend they book their tickets in advance, make a seat reservation and allow longer to get to their destination."
London Waterloo services - which have been disrupted for more than a fortnight as part of a major project - have been reduced even further until Tuesday.
Fewer than half of normal weekday services will operate as platforms 1-14 are closed to enable platforms to be extended to accommodate longer trains, boosting capacity at the UK's busiest station by 30%.
A Network Rail spokeswoman said: "We apologise to our passengers for the added disruption to their journeys and thank them for their patience.
"This all about delivering a bigger better Waterloo station fit for the 21st century."
National Express, the UK's largest coach operator, is putting on an extra 4,000 seats over the weekend to destinations affected by the Euston and Waterloo work.
Between Saturday and September 2 there will be no Southeastern services to or from London Bridge, Waterloo East and Charing Cross as work continues to rebuild London Bridge and the surrounding railway.
Queuing systems will be in place at Cannon Street station between Tuesday and Friday next week to cope with demand.
Due to Crossrail work there are no trains between London Liverpool Street and Shenfield, Ingatestone, Billericay and Barking on Sunday and Monday
Modernisation work means services between Cardiff and Newport will be reduced until September 3, while no trains will operate between Swindon and Bristol Parkway from Saturday until September 17.
Travel warnings have also been issued to motorists as traffic is expected to rise by a fifth compared with last year's summer bank holiday.
Some five million motorists are expected to embark on leisure journeys on Monday, up by nearly one million on the 2016 figure.
The RAC recommended drivers wanting to beat the queues this weekend should avoid Friday afternoon and evening, and set off early if travelling on Monday.
The proportion of Britons planning an overnight UK trip this weekend has risen by more than a third compared with last year.
Some 6.9 million adults are planning a domestic break over the last long weekend before Christmas, up from 5.1 million in 2016, according to tourism body VisitEngland.
Travel trade organisation Abta estimated 2.1 million British holidaymakers will travel abroad over the weekend with Spain, Turkey and Greece among the most popular destinations.
Heathrow expects 243,000 departing passengers on Friday alone, while an estimated 304,000 will fly from Gatwick over the entire weekend.