Network Rail are examining a 4ft section of wall from Roebuck Bridge on Corsham Road in Lacock, after debris fell from the bridge on to the embankment next to the railway today (February 27).
Contractors for Network Rail, Dyer and Butler, have erected barriers around the wall and put temporary traffic lights in place to create a single-file traffic management system while the work takes place.
The damage was reported by a passer-by at 7.49am, who noticed that a section of the wall had fallen away.
Network Rail sent a bridge examiner to the site shortly after the report came through, but authorised normal line speed past the location for trains.
They also diverted two First Great Western high speed services while it was established which bridge was affected, with both services travelling via Hullavington rather than calling at Bath or Chippenham.
Police closed the road to vehicles shortly after 8.30am, but it was re-opened with the traffic management system in place at 5pm. It is unknown how long the repair work will take.