The Inwood Iron Bridge is a Pennsylvania (Petit) through truss bridge fabricated by the Pittsburgh Bridge Company and erected by its agents Nelson and Buchanan in 1899. This one-hundred-sixty-foot-long, single-span, pin-connected steel bridge was determined eligible for listing in the National Register of Historic Places in 2007, and a distinctive feature is its Z-shaped plate floor beam hangers. It was closed because of structural deficiencies in 2006. The Pennsylvania Railroad pioneered the Pennsylvania (Petit) truss as a variation of the Pratt truss, adding half-length struts or ties to panels with braced diagonal members and using an inclined top chord for economy of material and greater strength in response to increasing railroad loads. Nelson and Buchanan, formed in Chambersburg, Pennsylvania, in 1883 by Thomas McDowell Nelson and Andrew Buchanan, became agents for the Pittsburgh Bridge Company and were central to its success in Central and Eastern Pennsylvania; Nelson later served as president of the company from 1896 until its merger with the American Bridge Company. By the early 1890s, steel was replacing wrought iron in American bridge construction, a shift accelerated in 1900 when J.P. Morgan consolidated twenty-eight major United States steel fabricators and constructors into the American Bridge Company, including the Pittsburgh Bridge Company. More broadly, metal truss bridges used components connected in triangles, and by about 1875 American metal truss types had evolved into standardized forms using built-up compression members, pin-connected eyebars, and riveted standardized rolled sections and plates that allowed low-cost shop fabrication, shipment of preassembled parts, and rapid erection with semi-skilled labor. Just north of the Inwood Iron Bridge, the Waterville Bridge, a lenticular truss built in 1890 by the Berlin Bridge Company of East Berlin, Connecticut, was moved in 1985 from Lycoming County to Swatara State Park for Appalachian Trail use over Swatara Creek and was determined eligible for the National Register in 1980 as a rare lenticular truss bridge with a Warren configuration for its web members. The lenticular truss, patented in 1878 by the Corrugated Metal Company, resembled a Pratt truss in its diagonals but had curved top and bottom chords that gave it a lens-like shape and used less material by combining metal arch, suspension, and truss engineering, and stone from its original truss abutments was refurbished and used on the relocated and rehabilitated span.