The railway bridge is one of several built in the 1970s which now needs to be strengthened to carry faster and more frequent trains.
New anti-collision markings will also be installed and it will be repainted.
A replacement bus service will be in operation for rail passengers travelling between Westbury and Frome.
The A362, which runs under the bridge, is also closed to traffic until 24 November but pedestrians are still able to walk underneath.
Frome Independent Market, which attracts thousands of people to its monthly events, has told visitors to expect increased traffic and to follow diversions.
Those travelling to the town from West Shepton and Bruton are being told to follow the A361 and then the clear diversion signs to the Park and Ride.
