Metropolitan West Side Elevated Railway Bridge was used for electric trains from 1895 until 1958. This was the only 4 track bridge in Chicago’s history.
Most sites seem to call it a Jackknife bridge for some reason however, this is clearly a Chicago Type, Double Leaf, Trunnion, Bascule Bridge. This description can be broken down as follows:
– Chicago Type describes a design that evolved from research done in the late 1890’s by the Chicago Bridge Department to find a patent free design that would accommodate both land and water based traffic;
– These bridges rotate vertically about trunnions (axles) on the river bank;
– A leaf is the portion of the bridge deck that spans the river. All of these bridges use two leaves to span the river – thus Double Leaf;
– Bascule describes a family of draw bridges that use counterweights to assist in the vertical movement of bridge leaf.