Gareth Delbridge, 64, and Michael “Spike” Lewis, 58, were hit by a Swansea to Paddington train on 3 July.
An interim report from the Rail Accidents Investigation Branch (RAIB) said a third worker came “very close” to being hit in Margam, Port Talbot.
The three were part of a group of six carrying out maintenance work.
The train driver made an “emergency application of the train’s brakes” about nine seconds before the accident, the report said.
It was travelling at about 50mph (80km/h) when it hit the track workers.
Planning paperwork indicated work was due to start at 12:30 to coincide with the planned blockage of the a line, but workers began at about 08:50.
Witness evidence suggested there was a widespread belief at the local maintenance depot there was “no need to wait” for the planned line closure, the RAIB found.
There was a “general lack of understanding” as to how the planning paperwork should be interpreted, investigators added.
“The RAIB’s preliminary conclusion is that the accident occurred because the three track workers were working on a line that was open to traffic, without the presence of formally appointed lookouts to warn them of approaching trains,” the report said.
The report said the plan for repairs provided “no clarity on the safe system of work”.
At the time of the accident, the six workers at had split into two groups – none of them were aware a train was approaching “until it was too late to move to a position of safety”.
Investigators said the victims were “almost certainly” wearing ear defenders because a noisy power tool was being used to carry out repairs and a “vital safety barrier” was missing in the absence of a lookout.
The RAIB said its investigation was continuing, but the factors outlined “created conditions that made an accident much more likely”.
Further aspects of the accident will be investigated including group behaviour, the planning and paperwork, safety auditing and the selection and training of track workers.
The RAIB’s initial report adds to Network Rail and Great Western Railway’s report, while British Transport Police said previously its investigation would “continue for some time”.
Speaking after their deaths, families of the men said Mr Delbridge, from Kenfig Hill, was the “most loving husband, father, brother and granddad” while Mr Lewis, from North Cornelly, was “loved by everyone”.