Two railway workers struck and killed by a train were wearing ear defenders and could not hear it coming, police believe.
The victims, aged 58 and 64, died after being hit by the passenger service while working on tracks near Port Talbot in South Wales on Wednesday morning, according to British Transport Police (BTP).
The pair were from North Cornelly and Kenfig Hill respectively, two villages several miles south of Port Talbot, and their families are being supported.
A third casualty was treated for shock at the scene but was uninjured.
Network Rail is facing demands for answers after their deaths and Transport Secretary Chris Grayling has said there will be an investigation.
Superintendent Andy Morgan, from BTP, said: "Shortly before 10am this morning, we received a report of a train striking multiple persons on the tracks here in Port Talbot.
"Our units were very quickly on scene, alongside the Welsh Ambulance Service.
"Very sadly, a 58-year-old man from North Cornelly and a 64-year-old man from Kenfig Hill were pronounced dead at the scene by paramedics.
"Following a number of urgent inquiries into this tragic incident, it has been established that the three people were railway workers who were working on the lines at the time.
"The initial stages of the investigation suggest that the two men who died had been wearing ear defenders at the time, tragically, could not hear the passenger train approaching."
Witnesses at the scene and on board the train will also receive trauma support, Supt Morgan added.
Union bosses have called for a full investigation into the deaths, which involved the 9.29am service from Swansea to London Paddington, a 10-carriage GWR Intercity Express Train, and one of 93 in the fleet.
Manuel Cortes, Transport Salaried Staffs' Association general secretary, said: "It's too early to speculate about what has happened here but clearly something has gone badly wrong.
"There must now be a full investigation because it is simply not acceptable that in the 21st century people go out to work and end up losing their lives."
Network Rail Wales route director Bill Kelly said the railway network owner was "shocked and distressed" by the "dreadful accident", and added that it was "fully co-operating" with investigators.
He added: "Our thoughts are with the families of our colleagues and our members of staff who will be affected by this tragic loss, and we will provide all the support we can."
The incident resulted in cancelled trains, with replacement buses being put on for rail passengers.
Investigators from the Rail Accident Investigation Branch (RAIB) and the Office of Rail and Road (ORR) have also attended.
Mick Cash, general secretary of the Rail, Maritime and Transport (RMT) union, said: "This is shocking news. RMT is attempting to establish the full facts but our immediate reaction is that this is an appalling tragedy and that no-one working on the railway should be placed in the situation that has resulted in the deaths that have been reported this morning.
"As well as demanding answers from Network Rail and a suspension of all similar works until the facts are established, the union will be supporting our members and their families at this time."
Supt Morgan added: "We have a number of officers who remain in the area and we are continuing to work alongside the Rail Accident Investigation Branch to understand the full circumstances of what happened in the moments before this incredibly sad fatal collision."
Constituency MP Stephen Kinnock said: "I'm very concerned to see reports of the tragic accident on the rail line between Bridgend and Port Talbot.
"This is awful news, and my thoughts are with the families of all concerned."