The Austerity Engine is a saddle tank engine. He is the same class as Wilbert.
In 1993, The Fat Controller wanted to buy a new tank engine to help Donald and Douglas. He borrowed Wilbert from the Dean Forest Railway to see if he liked the performance of his class. The test went well, but the attempt to purchase the new engine fell through.
Later, when Asbestos visited Sodor with the other famous engines, Richard Hatt decided that it would be good to let her work with Percy, as the little green tank engine had been disappointed by the failure to procure the Austerity Tank Engine