Photo: Gondolas With Structures For Hoover Dam (Circa 1930-1935)

Photo from the University of Nevada/Las Vegas:

https://special.library.unlv.edu/ark:/62930/d1779v

Photo can be enlarged.

First gondola is UP.

Bob Chaparro

Hemet, CA

We’re the empty flat cars used for more braking?

I’m only guessing but the empty flat cars may have been used to better distribute the weight of the steel structures going over a bridge. 
--
Charlie Duckworth 
Omaha, Ne.

As Bruce said, they're idlers. The girders are a bit longer than the cars' I.L.  Note that the gons each have an open drop end facing the adjacent flat. The cars are in groups of three, two gons with open ends facing opposite directions with a flat between, making efficient use of idlers.

Jack Mullen

Used to see MANY utility pole loads set up that same way.  Idlers used to allow shipping poles longer than the flat of gon.  




--
Ted Larson
Trainweb.org/MHRR   ---   GN in 1965   ---   NASG.org 

It is unlikely for weight.  The Boulder City Branch was laid with relay 90lb, with 23 ties per 39' rail length (although they used 31' rails) so was equivelent to any UP main line at the time.  The grades were 1.1 to 1.2%.  The line was super elevated to allow 45mph.  There are photos of the smaller challangers in Boulder City.  We believe that the larger chalangers with centepied tenders would have had issues on the wye.

Randy Hees,
Retired Director, Nevada State Railroad Museum, Boulder City

The flat car that is second in the cut of cars appears to be one of UP's class F-50-x early Harriman Common Standard flat cars, or one of SP's cars of the same Common Standard design (kits available from Owl Mountain).

Todd Sullivan.