A section of railway between Gloucester and Swindon 'had rock fall issues'
A boulder fell on to a railway line in an area of "known disturbance" where some minor rock falls occurred "from time to time", it has been revealed.
The rock tumbled on to the line near Stroud, Gloucestershire, on Monday.
The cutting has safety netting in place but Network Rail said it was not built "to stop anything of this size".
The company will now examine whether "more resilient measures" are needed, but said it was too early to say if it was a "close call or not."
"It's certainly something we're going to be looking at very closely in terms of how it happened, what the potential consequences could've been and how we could prevent anything like it from happening again in the future.
"We are going to have to ask ourselves about the geological knowledge we have of that particular cutting and do we need to urgently update that and put some more resilient measures in place?" it said in a statement.
Services were disrupted between Swindon and Gloucester with trains being diverted via Bristol Parkway.