While the Milwaukee Road had built a modern transcontinental railroad, they were late to the game, and the required traffic to remain profitable never materialized. The railroad ended up in a long slide until it went bankrupt and merged out of business in the 1980s.
The first bridge at this location was built in 1906 and was finished by April 1907. It was a temporary wooden bridge, and it was built to support the railroad construction west of the Missouri River. A permanent steel bridge was built in 1907 and opened on March 19, 1908. That bridge was in use until the Oahe Dam project was underway. Waters from the dam would flood the bridge, so the US Army Corps of Engineers built a replacement bridge in 1961. That is the current bridge that we see in these photos. The bridge is very strong and massive structure. It has two through truss sections. These through truss sections are arranged in a very unusual manner in that they are not adjacent to each other. Rather, there is two spans of deck truss between the two through truss spans. This allows two different navigation channels through the structure. Based on aerial photography, I estimated the bridge to be 1,820 feet long, with the through truss spans being 275 feet each. The northwest end of the bridge has a long causeway that I estimate to be 3,650 feet long. The combined length of the bridge and causeway is just over one mile.
Once the Milwaukee Road went under, the State of South Dakota purchased this rail line and bridge. It was later leased to, and then sold to the BNSF. The BNSF has a mainline running though Bismarck about 85 miles to the north. The Mobridge line is used for overflow traffic. At first, this was seasonal for grain shipments. In recent years, the demand for coal from the Powder River Basin in Wyoming has resulted in long unit trains using the Mobridge. The line is mostly single track with few side tracks. The BNSF found it was hard to operate trains in both directions. As a result, they adopted a operating mode where they switch directions every other day. They will run empty trains west one day, and they run full trains east the next day.
The city of Mobridge was named after the railroad bridge. The railroad bridge picked up this name after a telegraph operator at the construction site once signed off as "M. O. Bridge". Other telegraphers started referring to the site as M. O. Bridge, and then later Mobridge. The name stuck.