The Jackknife Bridge, Chicago, Illinois. (1907)

The Jackknife bridge, designed by William Scherzer and built in 1895, spanned the South Branch of the Chicago River between Jackson and Van Buren Streets. It had two side-by-side sets of double railroad tracks, which were used by Chicago elevated ("L") trains until the late 1950s. 

The two halves resembled a pair of gigantic steel face-to-face rocking chairs, which rocked back and away from each other when opening.