We noticed these gates when we were cycling along the Western Flyer cycle path, on a section between Ferndale and the Town Centre on what was previously a canal route, so stopped to take a closer look.  We understand they were originally an entrance to an area of the Great Western Railway works - possible Gate 24 (according to a member of the Swindon and its Changing Faces Facebook group).  The grounds were later occupied by Plessey Sheet Metal and later still Claires Equipment.

There is a wonderful photo of this gateway when in use in March 1953 at:  Flickr.com.

How sad to see them in their current state, but fortunately they are still there, not totally lost, waiting for passers by to notice and admire them.