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. He was designed by the Arlesdale Railway's own Chief Engineer, Ivan Farrier, working with Ian Smith of the Ravenglass and Eskdale Railway, who also needed a fourth steam locomotive. It is not surprising therefore that Jock and the R&E's "Northern Rock" should show marked similarities.
Jock's design incorporates all the best features of Bert, Mike and Rex. As in the case of Bert, the tall chimney and cab give Jock a true Narrow Gauge appearance. Except for the boiler, firebox and cylinders, which were fabricated at Crovan's Gate Works, Jock was built entirely at Arlesburgh Works. Stephen Hatt played a role in arranging Jock's construction, a project which was dear to his father's heart.
When Jock first came out for testing he was still unnamed. Jock's paint colour reminded Douglas of his days on the Highland Railway, and he suggested the name "Jock" for the new engine. The Small Controller and Jock liked the name so it stuck.
Later on, Jock pulled a doubleheader with Mike, who had come to resent Jock's popularity and resulting arrogance, and tried to teach him a lesson by cutting off steam, forcing Jock to do all the work. The prank backfired when Mike broke down on the homeward journey, and Jock had to pull both him and the train. That evening, the two engines made up their quarrel, and Jock resolved to be less conceited in the future.
Jock was very cocky when he was first built, as a result of being stronger and more popular than the other engines, but soon learned the value of teamwork when Mike's injector failed and he had to pull Mike and his train single-handedly.
Jock is painted in the Highland Railway chartreuse yellow livery with red lining. He has a brass dome, brass builder's plates on the sides of his cab, red nameplates with gold writing mounted on each side of his boiler and "ARLESDALE" written on the sides of his tender in yellow.