The S class proper were 31 locomotives built in 1914 by Baldwin Locomotive Works and numbered 6000–6030. The S-1 class comprised 50 locomotives numbered 6100–6149, and 25 locomotives numbered 6150-6174. They were built between 1923 and 1924, 75 by Baldwin and by Lima Locomotive Works . The Class S-1a were built in 1926. The first batch of 25 were built by Lima, and numbered 6175-6199. The second batch was built by Baldwin, and numbered 6200-6224.[1] They were nicknamed "big sixes" because until renumbering in 1954 their road numbers all began with 6.[1] Locomotives numbered 6009 and 6030 were scrapped in 1925, and their boilers used to build new Class T 4-8-2 Mountain Type locomotives numbered 5500 & 5501. 15 of the Class were retired and scrapped in 1938. The remainder of S-1/S-1a continued service until 1953-1959.