Police have confirmed that a 4ft section of wall from Roebuck Bridge on Corsham Road in Lacock was knocked down on to the embankment near the railway this morning (February 27).
A passer-by reported the damage at 7.49am and a bridge examiner is on site and has authorised normal line speed past the location.
At present, trains are being cautioned and Network Rail diverted two First Great Western High Speed Services while it was established which bridge was affected.
The two train services were diverted via Hullavington as a result of the bridge strike, not calling at Bath or Chippenham.
Police closed the road to vehicles shortly after 8.30am. Network Rail's minor works team will carry out repairs to the bridge.