A golden retriever known as Bramble has been missing since November 15.
The female dark golden dog was with her owner at Batleys in Blagrove Industrial Estate, near Junction 16 of the M4, when she ran off.
She got through the fence and on to the railway embankment and headed in the direction of Chippenham.
The five-year-old, who has been spayed, is very well fed and was not wearing a collar.
Anyone with information should phone Helen Smith on 07919615993.