Colonies of slow worms near the tracks of a new rail link could be why two of the stations will not be ready for business when the line opens.
Slow worms look like snakes, but are actually legless lizards
The £30m Ebbw Vale to Cardiff link has been under construction for the last year and is due to open in December.
But environmental organisations have discovered that the protected species live where two stations are planned.
Collectors are trying to find the worms to move them to a safer place, but cold weather is hampering efforts.
It could mean that two of the planned stations in Llanhilleth and Cross Keys will not be ready in time.
Project director Richard Crook said: "The presence of slow worms, a protected species, has caused some delays at two of the station sites, at Llanhilleth and Cross Keys.
"Removing slow worms can be quite a time-consuming process, and this has been compounded by the cold weather in the summer, which meant that they were not as active as normal.
"At present the Llanhilleth site is making good progress, so we are hopeful that it may be ready at the same time as the other stations.
Tracks along the line have been upgraded
"However, it is important to remember the overall scale of this project.
"To open a railway line and provide a new integrated passenger service with bus links so close to schedule is a major success, and may even be a record for the UK's railway system," he added.
The new train line will provide better links for people in Blaenau Gwent to travel to Cardiff and is being funded by the Welsh Assembly Government.
There has been some criticism that the line does not connect directly with Newport, and there have been delays with the opening, which had been expected earlier in the summer.
It has been 45 years since a passenger train last ran on the tracks, which have since been used by freight services.
The Ebbw Vale rail link scheme will upgrade 18 miles of railway line for passenger services, restoring a link with Cardiff that was severed in 1962.
The project is a key part of the continued regeneration of the area following the closure of the Corus works in 2002.