Since this thread is about suburban light rail in Chicago, it's almost certainly Metra. Metra is not Amtrak, though they do have their own police, as well. Here is most certainly how the encounter would go in my case if asked for ID.

I'd say no.

These are outdoor platforms or just ground-level sidewalks with no gates, barriers, and entrances, in many cases. There are no train personnel on the platform, no ticket windows, and since TSA has no authority to detain me. They would pretty much have to let me get on the train. I cannot think of any other option for them. Any LEO would be on the train not the platform. There would not even be a ticket to refund since I use a monthly pass.

I think it's just security theater.