According to The Island of Sodor: Its People, History and Railways, the people of the Arle Valley already had their own railway line which predated the Mid Sodor Railway by nearly two decades.
The mine owners of Cas-ny-Hawin had consolidated together with people in the Arle Valley to build a small 2ft. 3in. gauge tramway to connect with the Port of Arlesburgh. At first the tramway was horse-worked, but they sought to follow the Skarloey Railway's lead of adopting steam traction to work their line, and by 1866, had an unspecified number of steam locomotives of their own, but were used for the mineral traffic only. Passengers were carried on the tramway, for free at their own risk, and charged only for the carriage of their hats, attire and other personal belongings.
The Peel Godred Committee saw interest in upgrading the tramway and extending it into the mountains to connect with the town of Peel Godred, and after several meetings with the mining companies, the committee bought up the tramway, and upgraded it to Board of Trade standards to allow regular passenger-carrying services.
The work was finished in 1874 and the tramway was open to the public, with a road coach link to Peel Godred from Marthwaite provided until the construction of the rest of the route to King Orry's Bridge was completed by 1880.
The route of the old tramway eventually became part of the Mid Sodor Railway as their "Valley Section", and ultimately the Arlesdale Railway.
The Island of Sodor: Its People, History and Railways does not mention specifically how many steam engines the Arlesdale Tramway had, or what they looked like, but it can be assumed they were all scrapped by the time Duke and the other known MSR engines were put into service in 1880. On the other hand Duke running the entire railway by himself would be quite hard so theres a small possibility these engines were kept around until newer engines replaced them.