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Baldwin Locomotive Works: Evolution of the American Locomotive, 1907

Classification

Old Ironsides was a four-wheeled locomotive with the two rear wheels being the driving wheels. The next engine Baldwin built had six wheels: the front two non-driving wheels being replaced by a four-wheel swiveling "bogie" car which improved traction and weight distribution on the track. By ten years later, Baldwin had created an engine, the six-wheels-connected locomotive, which answered the questions of increased power, weight distribution, and traction on curves. All six wheels were drivers: the rear two fixed and the front four drive-connected yet assembled to operate as separate pairs in negotiating curves.

As the wheel arrangements of locomotives grew, a system of classification was established. Three sets of numbers, in the format NN-NN-NN, described the type and combination of wheels. The first NN value shows the number of guide wheels in front of the larger driver wheels, the second value the number of driver wheels, and the third the number of guide wheels behind the driver wheels. Old Ironsides' classification was 0-4-0.