Side view of the bridge
Photo taken by MnDOT in 1941; submitted by Jason Smith
License: Released into public domain
BH Photo #290612
And neither this or the Aulwurm bridge have been assigned the lally column category. I'll leave it to you guys, since it's your discovery and discussion.
This type of support system is referred to as a caisson system, in which the supports are mainly steel tubes filled with concrete (or, in some cases, concrete columns), with sparse bracing in between for stability. They are also referred to as "Lally columns". This was very common with multiple-span truss bridges for many decades from the mid-1800's till around 1920, mainly with the older pin-connected bridges. Type "Lally" into the search bar on the home page, and most of the bridges that turn up feature this type of support system. Hope this helps!
I am struck by a similarity in design of two very old multi-span pony truss bridges - this lost one, and the extant but derelict Aulwurm Drive Bridge near Chicago http://bridgehunter.com/il/cook/aulwurm-drive/ (Best photos at http://www.historicbridges.org/bridges/browser/?bridgebrowse...). Both bridges are shrouded in some mystery. Though the trusses themselves are different, the piers and bents (or lack of bents) look almost the same. Sturdy round concrete pairs of piers, crossbraced to one another only by light trusswork, in lieu of a big sturdy bent. Was this a common construction technique for very old multi-span pony truss bridges? Are there others like these two?