Formed in 1865 by Andrew Carnegie.
Sites 3
- Keystone Bridge Co. v Phoenix Iron Co. Full text of case before the U.S. Supreme Court in 1877. Concerns the infringement of two patents belonging to the company.
- Descriptive Catalogue of Wrought-Iron Bridges Facsimile of the company catalogue published around 1874.
- Historic Bridges of the Midwest Photographic records of three bridges constructed by Keystone.