A man in his 60s was killed in the UK's first head-on train disaster of the century on Monday night, and rail officials are trying to determine what caused it.

Following the crash between the Welsh villages of Talerddig and Llanbrynmair on the mostly single-track Cambrian line, another fifteen persons received medical attention.

The impact was gradual enough that neither train derailed, even though it seems like the train that was supposed to stop was unable to do so.

One line of inquiry is leaves on the line in this densely forested region. Considering leaves to be the equivalent of black ice on roadways, Network Rail spends millions removing them off rails in the fall.

Police and inspectors from the Rail Accident Investigation Branch (RAIB) will also be checking for a signals failure. The Cambrian line was used as the pilot for the introduction of digital signalling and train control systems that is designed to reduce the chance of human error.

Elwyn Vaughan, a local Plaid Cymru councillor, said: “There are dedicated passing places. The norm would be one train parks there, and the other passes. The fundamental question is, why that didn’t happen?

“There are a couple of theories. One element is there’s a new digital system to stop trains. So did that work, were there any issues with that?

“And the other one is, of course, leaves on the line, because there’s a steep incline here. There’s a deep cut into the rock face here and a steady incline, so once a train starts sliding it would continue towards the bottom.”

However, Vaughan said he had been told the line was cleared of leaves a few hours before the crash.

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

Passenger Jonah Evans, 25, said: “There was a train that was stopped and the other train couldn’t stop. And the driver couldn’t get out the way with a train in front.

“The driver ran in and sat on a chair and said brace yourselves, we’re about to hit a train. Someone lost their teeth, cracked ribs. Because the driver told us it was happening, we could kind of get ready.”

Anthony Hurford, who had been travelling to Shrewsbury, told the BBC: “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, 60mph, 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.”

A man living close to the railway line in Talerddig described hearing a loud bang and a grinding noise as he sat down for his evening meal.

Peter Carson, 67, said: “The noise lasted a few seconds. We have a lot of military aircraft going over so at first I thought it was a plane in trouble. I now realise it was the sound of the trains hitting the brakes.”

Before the crash, Transport for Wales advised passengers that its services were running at reduced speeds through Dovey Junction station, just west of the crash site, because the track was “extremely slippery”.

The Cambrian line was used as the pilot for the introduction of digital signalling and train control systems known as European rail traffic management system (ERTMS) and European train control system (ETCS).

In October 2017, temporary speed restriction data was not sent to four trains travelling on the coastal section of the Cambrian line due to a failure with the ERTMS system, though no accident happened.

Speaking at the scene on Tuesday afternoon, Ch Insp Jonathan Cooze, of British Transport Police, said: “Sadly, a man in his 60s has died following the collision. The circumstances leading to his death are currently under investigation.

“While formal identification has not taken place, he is not believed to be a member of rail staff.”

According to Network Rail, leaf fall on tracks every autumn can lead to a slippery layer on the rail.

Fourteen people were injured in a train collision in Salisbury in October 2021 after engineering work delayed the cleaning of rails.