Network Rail has warned that disruption between London and Bedford is expected to continue for a week following a fatal train crash, which it described as a "tragic, isolated incident".
A train driver was killed and about 100 people were injured when two East Midlands Railway (EMR) services to London St Pancras collided at about 17:15 BST on Friday near Bedford.
Police said 28 people remained in hospital, including nine in a critical condition.
EMR services to and from London St Pancras have been suspended over the weekend, while Thameslink has urged passengers to travel only if essential.
Network Rail said the line is expected to remain closed between Bedford and Luton for the rest of the week, until 28 June.
As a result, there will be no GTR services north of Luton and no EMR services south of Bedford, with a limited rail replacement bus service in operation between Luton and Bedford.
EMR said pre-planned engineering work meant all lines between Bedford and London St Pancras were already planned to be closed on Saturday and Sunday.
Following the incident, from Monday, no EMR services will run between Bedford and London St Pancras. Trains that usually start and terminate at London St Pancras will start and terminate at Bedford instead.
EMR added it will operate a reduced service on Intercity and Connect routes, although Regional services will remain unaffected.
One train per hour would run between Sheffield and Bedford, Nottingham and Bedford, and Kettering and Corby, but there would be no services south of Bedford.
EMR Luton Airport Express services will also not run whilst the recovery operation continues. Passengers travelling between Luton Airport Parkway and London St Pancras in either direction may use their ticket at no extra cost on Thameslink services.
Network Rail's East Midlands route director, Mark Budden, said: "We know passengers will want to understand when services can resume.
"We will provide updates as soon as we have a clearer understanding of the recovery work required and when the railway can safely reopen."
Ellie Burrows, Network Rail Eastern regional managing director, added: "This is a complex and challenging task, and our teams will be working tirelessly to reopen the railway so we can resume services between Bedford and London.
"While investigations are still at an early stage, current indications are that this was a tragic, isolated incident."
The crash was declared a major incident by the British Transport Police on Friday.
Passengers on board the Nottingham service described the collision as "bewildering and frightening".
Alistair Adamson, who was travelling on the 15:50 Nottingham to London St Pancras train, said he feared there would be an explosion after the impact.
"All of a sudden, a really big impact," he said. "I immediately felt myself lurching forward and saw my colleagues across from me getting thrown around."
Dr Peter Knapp, who was in the front carriage of the train that struck the other, said: "When I got up, I saw all of the chairs everywhere. It felt like I'd been in a bomb explosion.
"I saw people's bloodied faces, people's legs looked broken, and there was smoke everywhere."
Shola Mene said she heard a "big bang" and that "people flew from their seats", adding: "There was a lot of blood. A lot of people had facial injuries."
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