These updated USRA Light Mountains were built to accelerate freight traffic on a single track main line. The first ten were built by ALCO-Brooks in 1926, eight more in 1928 by ALCO in Schenectady, and the last three in 1930 by the Soo Line's Shoreham Works in Minneapolis. There were minimal differences in weight between the individual series. The tenders initially had a capacity of 10,000 gallons of water and 16 short tons of coal. These were later enlarged and some locomotives also received completely new, larger tenders.