Watch: Footage of the derailed trainpublished at 18:25 BST
BBC colleagues are at the scene and have sent back this footage of the derailed train.
Train derailed in upright position in Wickford, Essex
A passenger on board a derailed train in Essex tells the BBC it "swung wildly for about 10 seconds"
Emergency services are responding at the scene after a London-bound train carrying up to 45 passengers partially came off the tracks in Wickford - what we know so far
British Transport Police says "the rear carriages of a train derailed in an upright position" at 14:12 BST, adding there are no reported injuries - its statement in full
National Rail says major disruption is expected "until further notice"
It comes a day after a separate train derailment in Lewes, Sussex - the cause of that incident is still being investigated
Responding to the derailment, the Department for Transport says Britain's railways are "amongst the safest in the world"
Edited by James Hore, with Alex Stevanovic at the scene in Wickford and Lewis Adams writing
BBC colleagues are at the scene and have sent back this footage of the derailed train.
Train derailed in upright position in Wickford, Essex
Greater Anglia has said any tickets dated today will be valid for travel tomorrow, at no extra cost.
Those who are stranded have been asked to speak to a member of staff for assistance about onward journeys.
Tickets can be used on:
And via London Underground services:
Matt Knight
at Southend Airport
There are lots of confused passengers at Southend Airport right now.
Many of them can't get to their onward destinations from the railway station here.
We've heard an announcement saying not to buy rail tickets. People are hunting for taxis as there are no replacement buses.
One Greater Anglia official at the station said no trains would run tonight and it was likely the line would be shut all weekend.
Alex Stevanovic
Reporting from Wickford
Image source, Alex Stevanovic/BBCPeople living close to the scene have been handing out bottles of water and ice lollies to the emergency workers.
They're working in sweltering conditions: It's 33C here.
National Rail says major disruption is now expected to continue "until at least the end of the day" today.
Earlier it had said delays would be felt until 18:00 BST.
Kris Bramwell
BBC News
Image source, Jon YoungJon Young got off the train with a member of Greater Anglia staff who was on board
Jon Young, 37, from Brentwood was on the train that derailed and says everybody he's seen is OK.
He tells the BBC: "The train swung wildly for around 10 seconds, like it was about to tilt over."
"It then carried on going for a couple of minutes but there was a horrible vibrating sound, so something obviously wasn't right," he adds.
"Then the train then came to a halt and there was smoke coming from the back carriage."
He says that people "panicked" and ran up the train, before they "calmed down" and waited for help to arrive.
Young says emergency services helped him down from the train onto the side of the tracks.

Alex Stevanovic
Reporting from Wickford
Image source, Alex Stevanovic/BBCPolice are now putting up blue and white tape in Salcott Crescent as they look to secure the scene.
They are closing it at the junction with Pebmarsh Drive
Martin Frobisher, a director at Network Rail, says teams are working to establish what caused the derailment.
He says two carriages came off the track - but remained upright - and no significant injuries have been reported.
"We would ask people to avoid speculating on the cause of any incidents until further information is available," Frobisher adds.
Image source, Alex Stevanovic/BBCAaron Poyser, who lives next to the railway line, was in the garden when he heard an "almighty crunching sound".
"It’s almost like someone was shovelling stones. I saw this plume of dust and smoke," he says.
"You could see the train had come off the tracks. It was quite scary, to be honest."
Image source, PA MediaA Greater Anglia passenger train has derailed in Essex, the second such incident in the UK in two days. Here's what we know so far:

The Department for Transport has stressed Britain's railways are "among the safest in the world" as it reacts to today's incident.
"The train is upright, there are no reported injuries and investigators have already been deployed to the site," it says.
"Our railways remain amongst the safest in the world and while incidents like this are extremely rare, when they do occur, they are taken very seriously."
James Kelly
BBC Your Voice

Danielle Belton runs a business on an industrial estate near to where the train derailed.
She says the noise was so loud it shook her office.
"We're used to hearing the trains go past all the time, but we heard what sounded like a crash," says Belton, who is a Conservative councillor in neighbouring Rochford.
"There was a big, loud noise and then the unit rattled and sort of shook.
“All the girls in my office, we all kind of went, 'blimey, what was that?'. You felt the office rattle and you heard the loud noise."
Alex Stevanovic
Reporting from Wickford
"There are residents out in the road just looking at the train, says the BBC's Alex Stevanovic.
"Emergency services are also here, including plenty of police cars and ambulances.
"The train is behind a row of bushes so you can't quite see it properly from this spot in Salcott Crescent," he adds.
We've heard from Andy Barnes, the Conservative leader of Basildon Borough Council - in whose authority area Wickford is based.
"I would like to thank all of the emergency services who are currently responding to this incident," he says.
"Our thoughts are with everyone affected, and we sincerely hope that all those involved are safe and well."
Alex Stevanovic, a BBC reporter at the scene in Salcott Crescent, has sent these images.
They show railway workers, paramedics and police all responding to the derailment.
Image source, Alex Stevanovic/BBC
Image source, Alex Stevanovic/BBC
Image source, .By James Kelly and Richard Irvine-Brown
BBC Verify is analysing images from a train derailment in Essex.
Images show a Greater Anglia train with at least one carriage that has come off the rails in Wickford, Essex.
The derailment happened near an overhead walkway between Salcott Crescent and Russell Gardens, 1km (0.7 miles) east of Wickford station.
Images also show air ambulances landing in an open space a further 800m to the east, near Meresa Crescent.
Data from the website OpenTrainTimes shows the train was a Southend to Liverpool Street train, which left Rayleigh at 14:06 but failed to reach Wickford at 14:11 as planned.
The images above and below this post have been verified.
Image source, .There were about 40 to 45 passengers onboard the train, Basildon Borough Council says.
It was travelling from Southend towards London and "no major injuries have been reported", according to the local authority.
"Council officers are also at the scene and are liaising with partner agencies," a spokesman says.
"The council is in contact with emergency services and other partners and is providing support as required."
They add that an air ambulance landed at the scene as a precaution.
Images show air ambulance crews landing in a field near the derailed train in Wickford, Essex.
British Transport Police said earlier there were no reported injuries.
Stay with us for the latest updates.
Image source, No credit
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Anna Johnson
BBC East
An eyewitness tells me people were "in shock" at the scene of the derailment.
He says groups of people have already gathered in the area in response.
"A couple of people were in shock," the man says, speaking anonymously.
"No one was off the train when I was there - the train was just off the track.
"I was quite shocked. It's not something you see every day, or you hear about often - especially on the UK railway."
As a reminder, a train also derailed near Lewes, East Sussex, yesterday.