Jock, named by Douglas for his yellow-green livery, is the newest and strongest steam engine on the Arlesdale Railway.
After Rex's steampipe began leaking one day and Frank had to take his train, the Small Controller began thinking they needed another engine and so he arranged for Jock to be built. When Jock first came out for testing he was still unnamed. Jock's paint color reminded Douglas of his days on the Highland Railway and suggested the name "Jock" for the new engine. The Small Controller and Jock liked the name so it stuck. Later, he pulled a doubleheader with Mike.
Jock was very cocky when he was first built, as a result of being rather strong, but soon learned the value of teamwork when Mike's injector failed and he had to pull Mike and his train single-handedly.