I was not surprised when the ticket for the "L" in Chicago that I bought from a machine looked like this:
But I was a little surprised when I walked up to the gate and there was nowhere to insert it, just a "touch here" proximity pad for peoples fancy reusable tickets.
I knew it wouldn't work, but given no other option I held my ticket close to the pad and...the gate opened. I was not expecting that.
When I was on the train, I held the ticket to the light to find out how the magic works - and sure enough, embedded in the ticket is a little passive RFID circuit and my little paper ticket was also a fancy reusable ticket: