And I thought a good answer might show up on the Milwaukee Road Yahoo group-
According to official records of the CM&PSRwy as conveyed from C.F. Loweth, then Engineer and Superintendent of B&B, the date(s) in the railroad's Record of Constuction of the Missouri River Bridge, Mobridge, S.D. are as follows:
May 21, 1906 First materials towed by boat from Evarts to bridge site.
Sept 21, 1906 Tracks laid to Pontis
October 12, 1906 First excavation started but quickly stopped due to failure to finding loose shale at unexpected depths.
March 6, 1907 Excavation began again.
March 18, 1908 Steel Erection Finished
March 19, 1908 A C-1 engine, hauling 16 cars loaded with steel rails. was run
across the bridge, and passenger train No. 3 of March 19, 1908 also passed over the bridge.
April 28, 1908 All riveting on the bridge was completed.
August 18, 1908 Painting was completed.
Construction was initiated on a temporary bridge at the site on May 21, 1906 and
finished on April 21. 1907 only to be washed out on May 29, 1907. A replacement temporary bridge was started June 8, 1907 and it was finished on June 16, 1907 but was washed out on June 25, 1907. Another replacement bridge was started on July 5, 1907 and was completed on July 18, 1907. It remained in service until after the completion of the permanent bridge (3/18/08). It was then torn down and removed.