I believe that the photo of the date plate says 1924. Just a guess from looking closely at the pic...
Thanks for confirming the purpose of the inner tracks. I always suspected that's what those were, but never knew for sure.
The inner rails are guard rails, which keep a derailed train from taking out the entire bridge
I doubt that they ever did--if you're referring to the second sets of tracks that are inside the wider sets on the bridge, I believe that those inner rails serve as some sort of stabilizer for trains that cross bridges, but I'm uncertain how they work. I have noticed that many railroad bridges that I've seen have these inner tracks.
Did these bridges once hold narrow gauge?
I use to pickup Taconite pellets under this bridge for slingshot ammo. Also fished there too.