I suspect as many as a thousand companies may have made parts for railroads. Perhaps as many as two thousand.

If railroading was highly important to those manufacturers, then they usually included some variation of "railroad" in their company names. If railroading was that important, then I generally include those companies in the database for this catalog.

Many companies existed in a "gray area" where they neither included a variation of "railroad" in their corporate names nor offered compelling evidence of exclusive dependence on railroading.

Westinghouse Air Brake Co. is widely known to have been the developer of railroad air brakes. It is generally assumed that railroading and Westinghouse were interdependent.

Other brake companies may have been equally dependent on railroading, but are now quite obscure. As a rule, brake companies had many markets outside railroading. For instance, braking was used in all manners of industrial applications including fixed engines for belt drives and automobiles. Brakes were heavily used in the mining industry for controlling hoisting and transportation equipment.

In practice, Westinghouse is included among railroad parts companies by custom; other brake companies are excluded because they did not put railroading in their names.

The same goes for boiler manufacturers and foundries. Boilers were used in thousands of applications outside of railroading. Foundries built all types of metal devices, but not all built locomotives, drive equipment, or wheels.

In the absence of contrary evidence, I exclude companies that lack a variation of "railroad" in their names.