A train of the Baltimore and Ohio Railroad idles on a Bollman Bridge in this 1850s image. Invented by Wendel Bollman, a Baltimore engineer, the bridges were the first all-metal railroad spans in history.
The intricate metal suspension truss structure was faster and easier to build than stone bridges, and stronger than wood. All the parts could be pre-made in a factory and shipped to a location for assembly. One Bollman Bridge survives intact today, it carries hikers across the Little Patuxent River on the Savage Mill Trail in Howard County, Md.
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