The S class proper were 31 locomotives built in 1914 by Baldwin Locomotive Works and numbered 6000–6030. The S-1 class comprised 125 locomotives numbered 6100–6224. They were built between 1923 and 1926, 75 by Baldwin and 50 by Lima Locomotive Works (#6150–6199).
They were nicknamed "big sixes" because of their size and because, until renumbering in 1954, their road numbers all began with "6".
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