Unless incorporated as separate railroad companies, mining railroads are excluded from the database.

Mining operations often move tremendous tonnages and frequently depend on rail transportation. This includes mining operations as diverse as copper, iron, limestone and coal mining. In the majority of cases, company-owned railroad operations are restricted to small numbers of privately-owned locomotives working on privately-owned tracks.

A few types of mining operations moved large tonnages across non-company land. Noted operations included taconite in Minnesota, copper in Arizona, gilsonite in Colorado and borax in Death Valley. In most of those cases, companies separated their rail operations into closely-held corporations. Most such operations included some variation of "railroad" in their names, and are therefore included in my project

A few mining-related companies remain in the database when I have been unable to find confirmation of full company names or their corporate status.

A final reminder. Just because a company owned a bit of track and a couple locomotives does not make it a railroad company. If a certificate carries the name of a mining company, but does not include "railroad" or "railway" in the official corporate name, I classify it as a mining company.