The story
Keystone is best remembered for the Eads Bridge in St. Louis, completed in 1874, which survives to this day. A number of its works are listed on the U.S. National Register of Historic Places.
Carnegie sold his company, Carnegie Steel Company to J.P. Morgan in 1901.
Works include (attribution):
Fairground Street Bridge (built 1895), spans ICG RR yard on Fairground St. Vicksburg, MS (Keystone Bridge Co.), NRHP-listed
Hannibal Bridge or Kansas City Bridge, spanned the Missouri River at Kansas City, MO, (constructed 1867–9) for the Hannibal & St. Joseph Railroad. (Keystone Bridge Co.).
Nokesville Truss Bridge, NE of Nokesville on VA 646 Nokesville, VA (Keystone Bridge Co.), NRHP-listed
Oak Ridge Railroad Overpass, SW of Shipman on VA 653 Shipman, VA (Keystone Bridge Co.), NRHP-listed
Pine Creek Park Bridge, N of Old Dam Rd., over Pine Cr. Fairfield, CT (Keystone Bridge Co.), NRHP-listed
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Riverside Avenue Bridge, Riverside Ave. and RR tracks Greenwich, CT (Keystone Bridge Co.), NRHP-listed
Valley Road Bridge, Stewartstown Railroad, Stewartstown RR tracks over Valley Rd., Hopewell Township Stewartstown, PA (Keystone Bridge Co.), NRHP-listed
White Water Creek Bridge, Whitewater Rd. over White Water Cr. Bernard, IA (Keystone Bridge Co.), NRHP-listed
Windsor Harbor Road Bridge, Windsor Harbor Rd. at Rock Creek Kimmswick, MO (Keystone Bridge Company), NRHP-listed
Keystone Bridge, Over North Fork of the South Platte River for access to the Mountain hiking trail Bailey Co. (Keystone Bridge Company)
From Wikipedia’s article on Keystone Bridge Company, shared under CC BY-SA 4.0.3