GWR No. 12 is a Sentinel geared steam locomotive which was built for the Great Western Railway and delivered in 1926. Its Sentinel works number is 6515. It was equipped to work train vacuum brakes and to provide steam heat for passenger trains. Initially, it was based at Swindon and used to work trains on the Malmesbury branch. Later, it worked at Brentford Goods Yard. These trials were not a great success and the locomotive was withdrawn in December 1926 and returned to Sentinel in January 1927.