The class 221 was a successful design of Mountain locomotive that was built by Vulcan Foundry over a long period of 24 years. It had a long and sleek boiler with Belpaire firebox, combustion chamber and Robinson superheater. The drivers measured only three feet and nine inches or 1,143 mm. With cylinders of 19.25 by 24 inches, this led to a tractive effort of more than 30,000 pounds or 138 kN. The first four were completed in 1924, followed by five more in 1938. The last and largest batch of 15 was delivered in 1948.