This bridge was built by the C&NW around 1900 in order to extend its line to the coal fields west of the river. In 1884, the C&NW built a 64-mile branch from Belle Plaine to Muchakinock, to reach the coal mines operated by its subsidiary, the Consolodation Coal Company (of Iowa). By 1900, the mines around Muchakinok were exhausted, and Consolidated had acquired new coal lands southwest of the Des Moines River. The C&NW built this bridge as part of its effort to reach those new mines, centered on the new coal camp of Buxton, around 1900. Buxton, at its peak, was the largest company town in the United States and home of the largest United Mine Workers local in the country. The line was abandoned after the last of Consolidation's coal mines closed in 1927.
See this article for history of the line:
-- https://www.cnwhs.org/articles/1408841578.pdf
This says that the abandonment of the line from What Cheer to Consol (the end of the line beyond Buxton, at the last mine operated by the Consolidation Coal Company) was authorized by the ICC in 1935, although traffic stopped in 1934 when storms washed out bridges on Coal Creek (between the Des Moines River and Buxton).
The article also dates the opening of the line to Buxton to 1901. The bridge has to have been in place for this, and was probably built no earlier than 1900.