Two railway workers have died after being struck by a train on a section of track in south Wales.
British Transport Police (BTP) said a third railway worker was uninjured and is being treated for shock.
The incident happened between stations near Port Talbot shortly before 10am on Wednesday.
BTP Superintendent Andy Morgan said: "We are making a number of urgent inquiries to understand exactly what happened in the moments leading to this fatal collision.
"Very sadly two people have died and my officers are now working to identify them and ensure that their families are supported at this incredibly difficult time."
Manuel Cortes, Transport Salaried Staffs' Association general secretary, called for a full investigation into the deaths.
He 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.
"Our Network Rail members together with everyone else at the company do so much to keep our railways running smoothly. They must be able to do this in a safe environment.
"Safety on our railways is paramount and sadly, as today's tragic events show, it can never be taken for granted."
A passenger on the train, between Pyle and Port Talbot, told PA it had been stopped for more than an hour, with those on board being evacuated by the BTP.
I'm very concerned to see reports of the tragic accident on the rail line between Bridgend and Port Talbot. I understand that there may have been two fatalities. This is awful news, and my thoughts are with the families of all concerned.'
— Stephen Kinnock (@SKinnock) July 3, 2019
The passenger, who asked to remain anonymous, said: "Police have been on train checking we are OK.
"Everyone at this end seems OK. Staff have given free water and drinks to everyone and are passing through train regularly."
The Rail Accident Investigation Branch (RAIB) has been notified.
A spokesman said: "We are aware of an incident between Port Talbot and Pyle.
"Two inspectors are now travelling to the site to gather evidence."
Transport Secretary Chris Grayling: "I am deeply saddened to learn of this morning's tragic accident in Wales. My sincerest condolences to the family, friends and colleagues of those who lost their lives." 1/2
— Dept for Transport (@transportgovuk) July 3, 2019
Both BTP and the RAIB confirmed the incident involved railway workers.
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.
"Our thoughts are with those involved in this incident and their loved ones."
National Rail said trains may be cancelled or delayed by up to 90 minutes in the area, with replacement buses running on some routes.
Two people have sadly died after being struck by a train in South Wales.
A number of urgent enquiries are being made to understand exactly what happened. Officers remain on scene, check with @nationalrailenq for travel disruption.https://t.co/yryoge36XTpic.twitter.com/OvTmGyx3lw
— British Transport Police (@BTP) July 3, 2019
A Welsh Ambulance Services spokesman said two rapid response vehicles, two ambulances and two vehicles from the hazardous area response team were among the response team.
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."