Paul is a narrow gauge Diesel.
B-B Welsh Highland Railway Diesel
Paul originally worked at a diamond mine in Wales with some of his brothers until the Thin Controller needed a Diesel to help the work on his railway. Paul didn't do so well on his first day; insulting the engines, teasing Stephen, and crashing into Millie.
Paul at first was insulting to the steam engines, as his old railway mostly had Diesels and his driver kept telling him "You're the next step in the Industrial Revolution." Paul let this go to his head and he caused rocus on the Skarloey Railway.
He has since softened and is friends with some of the engines, despite being grouchy and sometimes still insults the engines from time to time. Unlike some Diesels, Paul does learn from mistakes and tries to get better. He also hates weedkiller.
Paul is painted black all over with a nameplate saying "PAUL" in bold letters in white on his cab. On his old railway, he was painted red.