Some more thoughts further on this,
there were three major Tunnel projects that could have been the recipients of those loads of Pipe for Ventilation purpose. The Big5 Tunnel and the Newhouse Tunnel at opposing ends of Idaho Springs and the Lucania Tunnel up on the Fall River. The Flatcars may just have been stored there for some time prior to unloading perhaps?
If these were indeed bound for the Lucania Tunnel I think that unloading in I.S. would make more sense given the teamsters originate from there. The Newhouse and Big5 had their own sidings, and also I haven't (yet)seen a close-up photo of the Big5 vent pipe entering the tunnel, there was a large half-round recess in the top of each Door visible in the post-closure view I have seen.
Photo copyright and courtesy of Colorado School of Mines.
From the Idaho Springs Historical Society Coll.
UpSideDownC
in New Zealand