More than four million trains across Britain will be rescheduled from Sunday in the largest timetable change for decades.
The number of alterations is seven times larger than normal due to the introduction of new trains and services following billions of pounds of investment.
The shake-up is designed to increase overall frequencies and reliability, but some passengers will find their regular journeys are no longer possible.
Some disruption is expected while trains and crews are redeployed over the coming weeks.
Many of the timetable changes are a result of the £7 billion invested in the Thameslink programme in the South East, including rebuilding London Bridge station, new trains and track improvements.
Departure times will change for every train run by the UK’s busiest franchise – Govia Thameslink Railway (GTR) – which consists of Southern, Thameslink, Great Northern and Gatwick Express.
📢â°🚉 A new timetable starts Sunday 20th May. We are committed to introducing the biggest change in a generation smoothly and to do this a small number of these changes will take place in phases over the weeks after 20 May.— Thameslink (@TLRailUK) May 19, 2018
â¹ï¸ https://t.co/mZVqe28bqi pic.twitter.com/z55cPOOkYo
There will be almost 400 additional GTR trains every day.
The new GTR timetable was developed from scratch and was designed to tackle existing issues by extending stop times at busier stations and increasing turnaround times at destination stations.
Some passengers in a number of locations complained they will be served with fewer or slower services, including in Hertfordshire, Bedfordshire, Kent, East Sussex and Surrey, where many pay several thousands of pounds for annual season tickets to London.
Emily Ketchin, founder of campaign group Harpenden Thameslink Commuters, claimed the operator is “slashing key Harpenden services by a third”.
GTR said it carried out the biggest consultation of its kind, receiving 28,000 responses over 18 months.
Slashing key Harpenden services by a third is not an enhancement - the only thing you should be spinning are train wheels. The fact that @TLRailUK think it acceptable to reduce services even further shows the contempt they have for people who rely on their service. #saveHPDtrains https://t.co/YDYM55G1GT— Harpenden Thameslink Commuters (@HPDCommuters) May 19, 2018
@Se_Railway why are all the thameslink trains through maze hill cancelled today? This is literally day 1 of the new timetable and youâve cancelled all of them. That is embarrassing. Why?— James Johnson (@james_JJ4217) May 20, 2018
These new train timetables are impossible to understand and the various websites showing ravensborne to st pancras as 'not a valid route', which is rubbish. Going to be pot luck whether ppl get trains tomorrow or not, I think #thameslink #southeastern @TLRailUK #mess— Heather Nicholson (@AitchNic) May 20, 2018
It says it will have space for an extra 50,000 passengers travelling to London in the morning peak each day, and 80 more stations will have direct services to the centre of the capital.
GTR chief executive Charles Horton said: “We are introducing the biggest ever change to rail timetables to significantly boost capacity on the UK’s most congested network.”
The new timetable arrangements were described as a “disgusting insult” to disabled passengers by the Rail, Maritime and Transport (RMT) union.
It said the latest instruction from GTR tells staff not to attempt to place people of reduced mobility on a train if there is a possibility of delaying the service.
A GTR spokesman insisted “we place a priority on making our services accessible to all” but said it cannot hold trains when people arrive at a station without enough time to board.
Delays to electrification work between Manchester and Bolton in addition to a shortage of diesel rolling stock mean many of Northern’s planned improvements have been deferred.
The train you are catching today will change next week as part of the biggest rail timetable change in decades - check out our chief exec @anthonysmithTF's blog on what it means for passengers â°🚉https://t.co/BylAUJODUX pic.twitter.com/nGZNBuIKZ1— Transport Focus (@TransportFocus) May 18, 2018
TransPennine Express (TPE) becomes the sole operator between Manchester and Huddersfield outside peak times from Sunday. This adds extra capacity but trains will only stop at every other station to save time, affecting passengers wanting to make local trips.
Some passengers from Levenshulme and Heaton Chapel in Manchester will also suffer as they go from four trains per hour into Manchester down to three in the off peak, with a gap between services of up to 49 minutes.
Anthony Smith, chief executive of passenger watchdog Transport Focus, said: “Having plenty of staff on the ground from the start and during the initial transition, when changes bed down, will be critical.”