A rail passenger died and 15 people were being treated for injuries in hospital after two trains collided on a rural line in mid Wales on Monday evening.

Witnesses described how people were thrown to the floor of the train and pictures of the scene showed part of one of the carriages crumpled in the impact.

Investigators were at the scene near the village of Llanbrynmair in Powys on Tuesday morning to try to establish why the trains had collided on the line, a single-track section.

The incident took place close to a passing loop on the largely single-track Cambrian line, where one train should stop and the other proceeds at low speed on a small loop of track. It appears that the train that should have stopped was unable to do so, but the collision was slow enough that neither train derailed.

A multi-agency investigation is under way. Leaves on the line will be one possible avenue of investigation. Network Rail spends millions clearing autumn leaves from lines because they are the equivalent of black ice on roads.

It is believed that some passengers suffered broken bones, although police said their injuries were not believed to be life-threatening or life-changing.

Anthony Hurford, who had been travelling to Shrewsbury, told BBC Breakfast: “The word that keeps coming to my head is just brutal really. Just going from, I don’t know how fast we were going, maybe 40, 50, 60 miles an hour, to nothing in the blink of an eye.

“Somehow my body bent the leg of a table and ripped it off its bolts attached to the wall. Suddenly I was on the floor with my laptop strewn ahead of me wondering what the hell had happened.

“We tried to stop at the lights. At the top of the hill there’s a signal that I guess would’ve been a passing place and for whatever reason the train wouldn’t stop.

“There must’ve been 30, 40 people from fire [service], there was British Transport Police who had come from Birmingham, there were three helicopters, people had come from north and south Wales as far as I’m aware. I was checked by three or four different medics.”

A joint statement from Network Rail and Transport for Wales said: “At 7.29pm on Monday 21 October, two TfW trains were involved in a low-speed collision near Llanbrynmair in Powys, mid Wales.

“Sadly, one passenger has passed away, and a number of other people are being treated for injuries at nearby hospitals. Our first thoughts are with the family and loved ones of the man who has lost his life, as well as all the other people involved in this incident.

“The Cambrian railway east of Machynlleth will be closed while specialist teams continue their investigations, and we urge passengers not to travel to this part of the network. We’re extremely grateful to the emergency services who attended the scene and helped our passengers and staff in challenging circumstances.

“We are working closely with other agencies, including emergency services, to understand how this incident happened and they will have our full support.”

Supt Andrew Morgan, of British Transport Police, said: “We can sadly confirm a man has died following this evening’s incident. We extend our deepest sympathies to his loved ones, alongside everyone else impacted and specialist officers continue to provide support.”

He said the BTP was working with emergency services and the rail industry “to understand the circumstances leading up to this collision”. He added: “Residents will see an increased policing presence in the area in the coming days as a result.”

Multiple emergency services, including the Welsh ambulance service, Mid and West Wales fire service, Dyfed Powys police and the HM Coastguard, have been working alongside railway workers at the scene.

The first minister of Wales, Eluned Morgan, said: “My thoughts are with all those involved in the rail incident in Powys. I would like to thank the emergency services for their response and have asked to be kept informed of any developments throughout the night.”

According to Network Rail, leaf fall on the tracks every autumn can lead to a slippery layer on the rail “equivalent to black ice on roads”.

Fourteen people were injured in a train collision in Salisbury in October 2020 after engineering work delayed the cleaning of rails. One of the trains skidded on leaves.