I actually came here looking for additional information on an item I saw in a non-rail forum. Since I didn't find anything I went to the Midwestern Forum. Nothing there either. So I'm back!
Anyway, back in March a UP West Line commuter had a nuclear stress test done during the workday. When he arrived at Ogilvie to catch his train home he apparently set off a nuclear sensor! (I believe he entered through the Clinton Street corridor.)
TSA agents then were able to track this gentleman not only to Metra Train 47, dpt Olgivie at 5 04 PM for Elburn, but to the coach he was sitting in and right to where he was seated.
The gentleman said at first he was not aware that he was the cause of the activity until he heard one of the agents mention "isotope." Here's a real time video of the search taking place: Link to news story with video embedded.
(Btw, surprisingly since the link is to the CBS local news site in Chicago, I have run this video a couple times and it does not run well. Sorry.)
I had no idea Metra or any other commuter agency had this capability. A friend who works for NY MTA has told me, "Oh you'd be surprised at some of the things that go on."
Yes I am!