Blister I and Blister II, so named for the problems they have when starting in cold weather due, are small twin Diesels who work on the Arlesdale Railway.
The Blisters were built at the R.A. Lister Works in Dursley, England in 1932 with open-sided cabs. When they arrived on Sodor in 1985, the volunteers of Arlesdale used parts to rebuild them with two 2-cylinder 12 h.p. Lister engines to rebuild them to a 15-inch gauge. They were repainted and fitted with a new cab and bonnet, in the style of a Lister Blackstone RM2. In 1989, they were refitted with two 20 h.p. Lister engines to improve performance. Both have full electrics, radios, and air brake systems for working light passenger trains in emergencies. They are used for maintenance and shunting duties and were originally brought to the line for construction.