The railway line between Westbury and Frome has been blocked following a fault with the signalling system, leading to the cancellation of services between the two stations.
GWR has confirmed that the issues have been caused by axle counters failing on the line on Friday, July 17.
Disruption is expected to continue until at least 8pm and replacement transport, including taxis, has been requested.
Passengers have also been advised to consider postponing their journey if possible.
A spokesperson said: “Due to a fault with the signalling system between Westbury and Frome the line is blocked.
“Train services running through these stations may be cancelled.
“Some axle counters, part of the signalling system, have stopped working in the Frome area.
“Axle counters tell the signalling system if a section of track has a train on it.
“If an axle counter stops working, trains need to stop at the affected signal so that the driver can speak to the signaller to confirm if the track ahead is clear.
“We are working with Network Rail to gather more information about this to understand how this will affect our services and how we can return to running our normal timetable as soon as possible.”