Colleagues of the 22-year-old man - who was from the Rhondda valley - watched in horror as the high-speed Silverlink train mowed down the 27-year-old man, killing him instantly.
He has not been named because police are still trying to trace his close relatives.
The shocked driver of the Euston to Birmingham train has told investigators that the Balfour Beatty sub-contractor stepped onto the track as he approached the team of engineers - and he was powerless to avoid him.
Two other tracks which the sub-contractors were working on when the tragedy happened, had been closed to all trains.
However, the slow tracks alongside the closed lines were still open to all trains and it is believed the contractor strayed off his track into the path of the train.
The eight employees were working two miles north of Cheddington railway station in Buckinghamshire, at Ledburn junction on the West Coast main line, near Leighton Buzzard.
He was killed instantly when the Silverlink 20.39 Birmingham to Euston train passed through the station at 2231 BST and hit him.
Investigation
The team of contractor engineers were preparing the tracks for the night's planned repair work when the tragedy happened.
A spokesman for the British Transport Police said: "A major investigation is currently underway.
"As I understand, it the contractor stepped into the live line and was stuck by the train.
"The driver had no chance, there was nothing he could have done.
"There were four tracks, the two fast ones were closed and two slow ones were open.
The driver has given a statement saying he approached Ledburn junction at line speed which is 90mph on the left hand side of the slow line.
He said he saw a small group of workers in the shadows and one was just two feet from his train. He saw the man, but was going too fast to avoid hitting him.
The train involved was taken out of service on its arrival at Cheddington Station and the driver was treated for shock.
The crash plunged all trains services in the Hertfordshire and Bedfordshire areas into chaos, with delays of at least 90 minutes reported.
The 23.24 Euston to Northampton service was cancelled.
A spokesman for Silverlink Trains said on Friday : "This incident involving a track worker near Cheddington station was a tragic event and everyone at Silverlink extends their sympathy to his family.
"The train involved was running according to schedule and was passing the site on time.
"It is not yet known exactly what led to the accident and Silverlink will be helping with the inquiry."
A spokesman for Balfour Beatty said: "This incident is subject to a full investigation and our thoughts are very much with the family of our contractor at this awful time."
A spokesman for British Transport Police, said there were no suspicious circumstances involved in the death which appeared to be a tragic accident.
"We are currently carrying out extensive inquiries along with the Health and Safety Executive.
"The man lived locally but we cannot identify him until all his family has been notified of his death. The police are still trying to contact his next of kin."