D40125, nicknamed Old Stuck-Up by James, is a rude diesel engine.
BoCo was to show the visiting diesel around, but when he saw steam engines in the shed he refused to go further and insulted the railway for keeping them in service. A furious James bestowed upon him the nickname "Old Stuck-Up".
The next day, Old Stuck-Up was about to leave when he remembered he needed refuelling. He attempted to use BoCo and Bear's part of the shed to be refuelled and cleaned, but slid on cleaning fluid on the rails and crashed into the back of the shed. He was sent home after being talked to severely by the Fat Controller; BoCo noted that while he couldn't hear everything, he didn't think it sounded particularly flattering.