Sounds like a very minor low speed derailment in the south throat. It looks like at the end of the platform on Track 12. That location would probably foul westernmost tracks on south concourse, least tracks 2 through 16 and possible up to track 22 depending on how long the train is and its routing through the plant. That's easily all the tracks BNSF uses, which are like 2 - 10, IIRC.
I was waiting to board 7/27 at the time and we weren't given much info inside. I forget what train they were boarding at the time (possibly the Eaglette?) but they told everyone to go back to their seats in the Great Hall and wait.
It's likely unrelated, but there was a switch issue preventing them from moving Metra trains onto the south concourse two days prior (when I arrived on #8).