Baltimore and Ohio Class S comprised two classes

Class (locomotive)

Class refers to a group of locomotives built to a common design for a single railroad. Often members of a particular class had detail variations between individual examples, and these could lead to subclasses. Sometimes technical alterations move a locomotive from one class to another...

 of 2-10-2

2-10-2

Under the Whyte notation for the classification of steam locomotives, 2-10-2 represents the wheel arrangement of two leading wheels on one axle, usually in a leading truck, ten powered and coupled driving wheels on five axles, and two trailing wheels on one axle, usually in a trailing truck...

 locomotive

Locomotive

A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...

s.

The S class proper were 31 locomotives built in 1914 by Baldwin Locomotive Works

Baldwin Locomotive Works

The Baldwin Locomotive Works was an American builder of railroad locomotives. It was located in Philadelphia, Pennsylvania, originally, and later in nearby Eddystone, Pennsylvania. Although the company was very successful as a producer of steam locomotives, its transition to the production of...

 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

Lima Locomotive Works

Lima Locomotive Works was an American firm that manufactured railroad locomotives from the 1870s through the 1950s. The company took the most distinctive part of its name from its main shops location in Lima, Ohio. The shops were located between the Baltimore & Ohio's Cincinnati-Toledo main line...

(#6150–6199).

They were nicknamed "big sixes" because of their size and because, until renumbering in 1954, their road numbers all began with "6".