Clinton Rail Bridge

Located between Iowa and Illinois, the Clinton Swing Bridge is one the heaviest used railroad bridges across the Mississippi. The swing span is the largest ever erected, and it was the first electrified swing bridge.
When built, the bridge replaced an older single track bridge. That bridge contained several spans, mostly mismatched Pratt Trusses that would later be relocated to Fall River and Lawrence County, South Dakota.
This bridge currently contains four spans. A deck girder span, a Swing Span, a Parker Through Truss span and a Quadrangular Through Truss Span.
The Parker Through Truss contains 7 panels with riveted connections, the Quadrangular Span has riveted connections and the swing span has a pair of 8-panel leafs with riveted and pin connections.

Blueprints of the bridge, from Railroad Gazette: Volume 48 Issue 2

Unfortunately, the bridge is scheduled to be replaced in 2022, with demolition occurring by 2025.
As a result, this bridge is almost certainly bound for destruction when the new bridge opens, approximately 100 feet to the south. Construction on a new bridge begins in 2021, and the old crossing will be removed in 2025.
The bridge sits on all masonry substructures. The abutments are extra wide for a third track, which was where the old bridge sat.
During the first decade of the 20th Century, the C&NW engineered several trusses of their own and had them fabricated. The main span of this bridge was one of them.

Historic photo of the previous bridge

While not the oldest bridge across the Mississippi River, the series of three bridges which makes up this crossing is one of the largest. As a result, the author has ranked it as being of high significance, due to the unique design, massive size and old age.
The photo above is an overview. The photo below is sway bracing on the swing span.