Here in Minnesota, the temperature can drop down to -40F in the winter and over +100F in the summer. This temperature change does place a horrible strain upon the structures in the State. Having a highway suddenly buckle and thrust upward almost six inches, has happened rather frequently in the summer.
This was an old bridge, and the temperature changes finally caused something in it's basic structure to fail.
But what the heck, we have a new choo-choo train!
Hip hip hooray.
Half the reason there were so many cars there I bet is because that choo choo train has screwed up the freeway in that area. I tried to turn around the other week and ended up going north on 35w thinking I could get back to south 35w by just going off an exit ramp and crossing the overpass, you know... but the southbound entry ramps were all closed just about up to Roseville.
My thought...if one part of the deck was removed for replacement, the sun would shine directly on the steel beams, and the heat would be higher than normal on the steel, causing greater than normal expansion of the steel.
That, combined with the South pier now being canted toward the river (see pictures on other comments) consistent with erosion under the river side of the pier just below the lock,to me means that they need to examine multiple causes joining together in a “perfect flaw” to cause it to fail.