UP Sunfish Slough Bridge

• 1857: 44 miles completed from Clinton to Wheatland, Iowa by the Chicago, Iowa and Nebraska Railroad
• 1858: 20 miles completed from Wheatland to Lisbon, Iowa by the Chicago, Iowa and Nebraska Railroad
• 1859: 17 miles completed from Lisbon to Cedar Rapids, Iowa by the Chicago, Iowa and Nebraska Railroad
• 1861: 41 miles completed from Cedar Rapids to Belle Plaine, Iowa by the Cedar Rapids and Missouri River Railroad
• 1861: 41 miles completed from Cedar Rapids to Chelsea, Iowa by the Cedar Rapids and Missouri River Railroad
• 1862: 29 miles completed from Chelsea to Marshalltown, Iowa by the Cedar Rapids and Missouri River Railroad
• 1864: 29 miles completed from Marshalltown to Nevada, Iowa by the Cedar Rapids and Missouri River Railroad
• 1865: 29 miles completed from Nevada to Boone, Iowa by the Cedar Rapids and Missouri River Railroad
• 1867: 150 miles completed from Boone to Council Bluffs, Iowa by the Cedar Rapids and Missouri River Railroad
• 1867: 6 miles completed from Missouri Valley to California Junction, Iowa by the Cedar Rapids and Missouri River Railroad
• 1884: CI&N and CR&MR officially acquired by Chicago & North Western Railway
• 1887: 6 miles completed from Beverly to Otis, Iowa bypassing Cedar Rapids by the Linn County Railway
• 1887: Linn County Railway acquired by Chicago & North Western Railway
• 1891-98: Second track constructed between Clinton and Cedar River
• 1898-1900: Second track constructed between Cedar River and Mashalltown
• 1898-1901: New alignment completed between Boone and Ogden, Iowa; including the Des Moines River Bridge
• 1900-02: Second track constructed between Marshalltown and Council Bluffs
• 1933: Old route via Moingona between Boone and Ogden abandoned
• 1995: Chicago & North Western purchased by Union Pacific Railroad
• 1995-Present: Union Pacific operates the Clinton Subdivision between Clinton and Boone
• 1995-Present: Union Pacific operates the Boone Subdivision between Boone and Missouri Valley
• 1995-Present: Union Pacific operates the Omaha Subdivision between Missouri Valley and Council Bluffs

This structure is the easternmost crossing of the Mississippi River at Clinton.
Located entirely in Illinois, the bridge features 11 deck girder spans, resting on steel caissons and stone abutments.
Built in 1909, the bridge is in a deteriorated condition from how heavily used it is. As a result, it is scheduled for replacement. As of January 2018, the project is in early stages of construction, including surveying and soil tests.
The double track bridge currently carries approximately 100 trains a day. Since its original construction, it has been carrying this level of traffic nearly every day.
Overall, the bridge remains in a deteriorated condition. Preventative maintenance appears to have ceased due to the planned construction of the new crossing, which will be located nearby to the south.
Upon completion of the new bridge, the old bridges will be removed, ending their 110+ year history. It is expected that the old bridges will be removed in 2025.

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

The author has ranked this bridge as being moderately significant, due to the large scale version of the deck girder design.
The photo above is an overview. It can be accessed from the island on the west end of the bridge.