A little reading on a nearby bridge gives the date, 1886, and explains the names.
My guess would be that it was built in the late 1800s. All I know is that CKW built the route, and that the CKW was bought by the ATSF around 1901.
Any idea how long ago the RR was built? The 1903 KS RR map also shows AT&SF but I don't know anything about when this line was built or by whom. I don't have much access to RR history, so your info source would be interesting to look at. I'm always curious what else might be out there undiscovered.
The bridge would have been built by the Chicago, Kansas and Western Railroad. http://bridgehunter.com/ks/chautauqua/ckw-cedar-creek/
The RR was shown as the AT&SF on the 1936 county highway map.
The photo showing a pier in the middle of the river, just north of the Cedar Creek Bridge is from a railroad bridge and is the same railroad that built the stone arch bridge, just west of Elgin that is pictured elsewhere on this site. The railroad was removed in the late 30's. There are also stone abutments on either end to the east and west of the pier in the middle of Cedar Creek.