If you "connect the dots", you can see where the old railbed continued west where the road turns south, to cross the river.
One of the bridges that crossed Honey Creek failed under a locomotive the fateful night when Kate Shelley crawled across the Des Moines River bridge on this line to stop the next train.
1965 Boone West, IA topo shows old RR bed:
http://www.archive.org/download/usgs_drg_ia_42093_a8/o42093a...
1914 Boone, IA 15' shows Chicago and Northwestern:
http://www.lib.utexas.edu/maps/topo/iowa/txu-pclmaps-topo-ia...
If this was built 1920, then it is a railroad bridge. The railroad abandoned the line through here in 1931. The bridge contains a 13 foot wide deck, a size comparable to other railroad pony trusses.
Almost sure it is a former RR bridge too. Either that or it was imported from elsewhere.
http://ortho.gis.iastate.edu/client.cgi?zoom=1&x0=424257&y0=...
This bridge was demolished and replaced this past summer.