Katy Austin
Transport correspondent

The Rail Accident Investigations Branch (RAIB) says it will provide a further update "next week once we know more".

A spokesperson confirms its inspectors are on the scene and will continue to gather evidence at the site of yesterday's derailment at Lewes.

The evidence includes examining the accident site, including the derailed train, and railway infrastructure.

They’re collecting and reviewing evidence such as CCTV footage, signalling data, and data downloaded from the train.

They added the RAIB "will conduct a full, independent safety investigation into the accident."

The line between Haywards Heath and Lewes is closed "until further notice", but some trains are still operating through and stopping at Lewes station.

A second line between Brighton and Ore via Lewes is open, with trains mainly running to schedule. Southern says it has added additional carriages to some services to help with capacity issues.

As well as replacement buses, ticket acceptance is in place on alternative routes, some with other operators.

Passengers have been speaking to BBC Radio Sussex about what it was like being inside the train as it derailed.

Nick Johnson
Reporting from the scene

Image source, Jack Valpy/BBC

From this side of the track, you can clearly see how the force of the derailment has impacted the embankment and the perimeter.

Trees have been flattened and a piece of track is strewn across the embankment.

An incident tent has been set up by the rearmost derailed carriage. People in hi-vis jackets and white hard hats are walking between the tent and track.

On the other side, preparations are appearing to be made for a crane, which will have to be big enough to remove all the derailed carriages.

Katy Austin
Transport correspondent

It is a fact that extremes of heat can affect the railway. For example, we’re seeing services between London Waterloo and the West Country disrupted at the moment, because clay soil on embankments have shrunk in the prolonged dry conditions.

Plans are put in place in hot weather to manage risks. For example, speed restrictions, or even cancelling services if necessary.

But crucially, we simply can’t know at this stage whether the hot conditions were a factor in the Lewes derailment.

Rail accident investigators will likely be looking at a number of things, including the condition of the tracks and what’s under them. Plus, they’ll carefully analyse what data from on board the train shows about things like whether/when it braked.

Image source, Shannon Jones

Tracy McGregor, who was on the train, says it is a "miracle" nobody has life-threatening injuries as "it could have been really different".

She says she woke up this morning expecting to be fine, but was still feeling "quite shaken by it".

"It was like this metal tube carving through the earth, honestly, and it was just picking up dirt from the field," she says.

"So it ended up, when it did stop, there was this almighty cloud of mud and earth and everyone had mud on their faces.

"So it was quite a scene. It was really quite dramatic.”

The Southern Rail train derailed on the line between Haywards Heath and Lewes in East Sussex shortly before 16:00 BST on Thursday.

Lewes, in East Sussex, is about 7 miles (11km) from Brighton and 14 miles (22.5km) from Eastbourne.

The train was the 14:24 BST service from London Victoria to Eastbourne.

It had stopped at Clapham Junction, East Croydon, Gatwick Airport, Haywards Heath, Wivelsfield, and Cooksbridge.

The train was approaching Lewes when the incident happened.

It was due to call at Polegate, Hampden Park, and Eastbourne.

Earlier today, John Whitehurst - the interim CEO of Greater Thameslink Railway, the company that runs Southern Rail - spoke to BBC Radio Sussex.

You can listen back to his interview on BBC Sounds here.

Richard Wilcock, industry editor of Rail Magazine, says: "The first port of call for the rail accident investigation branch will be to look at the embankment and the actual track itself.

"It's hard to know when you look at those images, without speculating too much, where the issue arises, but it's clearly a buckled track and clearly heat, as we know, stresses the track."

An investigation is under way into the cause of the derailment, with various authorities, including the Rail Accident Investigation Branch and the Office for Rail and Road at the scene.

Wilcock says he would be surprised if there was not a temporary speed restriction in place at the time of the incident.

He says he expects some services in the area to be restored today, but says getting the carriages off the tracks will take "at least a couple of days".

Passenger Rachel Heath took this video moments after the 14:24 from London Victoria to Eastbourne left the tracks near Lewes.

Mark Norman
South East health correspondent

The BBC understands the two passengers with serious injuries were taken to the Royal Sussex County Hospital in Brighton and remain in hospital today.

Other passengers were taken to a number of local hospitals including the Princess Royal in Haywards Heath.

At its peak the South East Coast Ambulance Service had more than 50 staff at the scene, including its Hazardous Area Response Team and critical care paramedics, with more than 20 in its Emergency Operations Centre in Crawley coordinating their response.

Katy Austin
Transport correspondent

British Transport Police (BTP) led the initial response when the carriages left the tracks.

Now, things will be transitioning to a recovery phase, with Rail Accident Investigation Branch investigators doing their work, gathering evidence.

BTP will still support with things like searches on the scene, cordons, etc.

The Office for Rail and Road (ORR) sent investigators as well, but this is a standard procedure as they have a different role as the safety regulator.

George Carden
reporting from the scene

Image source, George Carden/BBC

Emergency service crews, Network Rail and the Rail Accident Investigation Branch have set up a command centre in a nearby field.

Image source, Rachel Heath

More now from Rachel Heath, who spoke to BBC Radio 4’s Today Programme this morning.

She says: "The train just started jostling, kind of bumping up and down, and then jostling from side to side like a seesaw, and everyone was screaming. I just remember looking around and seeing so much panic and fear in so many people’s eyes.

“Then all of a sudden it felt like we had almost lifted into the air, and then came back down and the train just fell on to its side, and there were a lot of children on board and all of the windows had broken. Just a very, very harrowing experience.”

Ms Heath says: “I just remember thinking to myself that I was going to die. There was about two seconds where I genuinely thought this was it, and so I went on my phone, immediately called my partner as the train was still flipping, and was just kind of in my mind as I made that call – calling to say goodbye.

“It’s just absolutely terrifying. Nothing like I’ve ever experienced, and nothing like you would ever expect to experience.”

What we know:

• A passenger train derailed in the area between Haywards Heath and Lewes in East Sussex shortly before 16:00 BST on Thursday.
• Photos from the scene show three carriages of the train on its side, with emergency service vehicles surrounding it, and people standing on top of the sideways tilted carriages.
• About 150 people were on board the train, with two people sustaining serious injuries and a further 18 with less serious injuries.
• All of the passengers were evacuated from the train.
• The Rail Accident Investigation Branch has arrived at the scene.
• Trains are unable to run between Lewes and Haywards Heath.
• An image from the scene shows what appears to be a buckled track.

What we don't know:

• When train services will resume on the section of track.
• What serious injuries the passengers have.
• What could be done to prevent this happening in the future.
• What caused the train to derail and when will the investigation conclude.

George Carden
reporting from the scene

Another Rail Accident Investigation Branch vehicle has arrived at the scene close to Lewes in East Sussex.

I can see at least 30 people from various authorities at the scene.

Police have been handing out water in these very hot conditions

Image source, Rachel Heath

Rachel Heath says she caught the affected train, the 14:24 Southern service from London Victoria to Eastbourne, but she tells BBC Radio 4's Today programme: "I wasn’t even meant to get that train I was meant to take the one before."

She says: "Everyone was screaming and I just remember looking around and seeing so much panic and fear in people’s eyes, and all of a sudden it felt like we’d been lifted into the air and then came back down and the train just fell on to its side.

"Getting off took a very long time – there were some passers-by who came and stood on top of the carriage and people were trying to open the emergency doors – eventually the train operators came and helped us get out."

Image source, George Carden/BBC

Todd Gander, who works at a nearby Londis, says “everyone did their very best to help” as locals bought supplies for those affected and gave people lifts.

He says emergency services crews came into the shop to ask for help and staff “didn’t hesitate”.

“We helped with anything and everything, and we also had the locals come down and ask if they could help,” Gander says.

“We sent them loads of water. Any customers who wanted to help bought loads of water [and] we gave them a discount.”

The Southern Rail website shows all services between Lewes and Haywards Heath have been cancelled for the next few hours.

Rail replacement buses are operating in place of services, also calling at Plumpton and Cooksbridge.

Southern Rail says disruption is expected to continue "until further notice".

Ticket acceptance is in place on other routes, which can be found here.

Rozina Sini
BBC News

Image source, Rachel Heath/BBC

Rachel Heath, from London, was on the train when it derailed.

"The train started rocking violently and then I remember clocking eyes with this guy and then there was a lot of screaming,” she says.

"It felt like we flew in the air and the carriage landed on its side and it slid.

"I felt like it was going to completely flip over.”

Heath says she saw people bleeding and the incident felt “like a movie”.

"When we flipped I was on the right and I fell on to the left side and I hit my head on the table,” she says.