Eighteen people were injured, including two seriously, after a passenger train derailed in southern England on Thursday, trapping passengers inside carriages that had toppled onto their sides.

South East Coast Ambulance Service said two people “sustained serious injuries,” while a further 16 with “less serious injuries,” were taken to hospital for treatment and further care.

Ambulance, fire and police services were deployed in large numbers to the scene of the derailment near Lewes in East Sussex, about 45 miles south of London.

British Transport Police Assistant Chief Constable Ian Drummond-Smith said, “We’ve declared a major incident and a significant emergency service response is ongoing at the scene.”

Aerial video showed dozens of first responders working around the derailed train, surrounded by emergency vehicles, with three air ambulances on the ground nearby.

Drummond-Smith said “all passengers on the train have now been safely evacuated.” There were about 150 people on board.

It was not immediately clear what caused the last three carriages of the Southern Rail service from London Victoria to Eastbourne to leave the tracks.

Images posted to social media appeared to show the train carriages on their sides, with people standing on them.

Passengers who didn’t require hospital treatment were taken to a nearby community center for additional support. Yana Ludwig, 56, from the United States, described the crash as a “completely terrifying” experience.

“There was a banging. Everything just went topsy turvy,” said Ludwig, who was traveling on the train from London to Lewes, according to Britain’s PA Media news agency. “It seemed like a really long time but it probably was 10 seconds of that before the carriage actually fell on the side.”

Ludwig recalled being propped up over another passenger, adding: “I’m pretty banged up.” She said travelers were checking on each other as they waited for emergency services to arrive.

East Sussex Fire and Rescue Service said it was working with other emergency services at the scene and advised the public to avoid the area.

“We have ten fire engines, a Command Support Unit, technical rescue units, officers and other resources at the scene,” the fire service said. “We have been helping reach passengers and assisting other emergency services.”

UK Transport Secretary Heidi Alexander said she was “deeply concerned” by reports of the derailment and thanked emergency responders working at the scene to support those impacted.

“We are working quickly with the rail industry and local partners to assist passengers,” she added.

The Rail Accident Investigation Branch, the UK body responsible for investigating rail crashes, said it had deployed a team of inspectors to the site to begin gathering evidence.

Lucy McAuliffe, Network Rail’s route director for Sussex, said the railway between Haywards Heath and Lewes is likely to remain closed for some time as specialist teams carry out assessments.

“I understand that people will have lots of questions about what has happened today,” she said, according to PA.

“I would like to reassure everyone that we are working closely with the Rail Accident Investigation Branch, who are on site to establish exactly what has happened.

“At the moment, it would be inappropriate to comment as to the cause of this incident but more information would be provided as soon as we can.”

The derailment comes amid a heatwave in the UK, several train services had been canceled and others were running at reduced speeds because of the severe heat. Temperatures in some parts of southern England reached 35 degrees Celsius, or 95 degrees Fahrenheit on Thursday.

Network Rail has previously cautioned that high temperatures can cause rail track to expand and possibly buckle.

“When steel gets hot it expands… If there is no room for the rail to expand, the rail can buckle and we need to close the line to fix it before trains can run again,” Network rail explains in a statement on their website.

The TSSA transport union also expressed its concern over the derailment. “With a large number of passengers involved, their safety and wellbeing must come first. We thank all emergency services and rail staff responding at the scene and await to hear more,” it said in a statement.

Train operator Southern Rail has advised the public not to travel with all lines through Lewes railway station shut down to allow emergency team, rail engineers and other specialist staff “to assess the situation and ensure the railway is safe.”

Thursday’s incident is the second serious train crash in the UK in two months. In June, two passenger trains collided in south-central England near the town of Bedford, killing one person and injuring dozens more. (CNN)