British Transport Police said on Friday it was responding to reports of a collision involving two trains about 60 miles north of London, with media reports saying the major incident had resulted in multiple injuries.
A video posted on social media showed what appeared to be the front of one train entangled with the back of another, with both still on the tracks.
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The East of England Ambulance Service said it had sent multiple resources, including an air ambulance, to the collision on the railway south of Bedford, and urged people to avoid the area.
Transport minister Heidi Alexander said in a post on X she was “deeply concerned” by reports of the collision.
Bedfordshire Fire and Rescue Service said crews were at an incident on the railway just south of Bedford and also called on the public to steer clear.
Train operator East Midlands Railway said it was unable to run services in or out of London for the rest of the day, while Thameslink said all lines between Luton and Bedford were blocked due to a problem under investigation.