In 1909, the Southern Pacific ordered two class MC-1 Mallets from Baldwin in the 2-8-8-2 wheel arrangement, which were the heaviest locomotives in the world at the time. They were intended to haul ever heavier freight trains across the Sierra Nevada. In the test runs with the two machines, the aim was to gain experience for later series production.

Since the Schmidt superheater had not yet fully established itself, a different design was chosen for the very large boiler. First there was a feedwater heater with an area of 1,221 sq ft. The high-pressure cylinders were then operated with saturated steam, which was passed through a 655 sq ft reheater before being passed on to the low-pressure cylinders.

During operation, it quickly became clear that the smoke produced by these powerful locomotives in the many tunnels posed a serious danger to the crews. Thus, the following classes from the MC-2, as well as the AC series, were built as Cab Forward. The two MC-1s were also converted into MC-2s in 1923. In 1931 they were converted to simple expansion and scrapped in 1947 and 1948 respectively.