Very cool - much mention is made of the negative aspects of such tracking, but we don't hear much of the positive side.
When I still lived in London my Oyster card (similar to a metrocard, it seems) saved me once, though not because of its tracking facility. I was miserable for various reasons and had gone for a loooong walk through the city, and just when I was really tired I was picked up by a man who bought me a drink, then another, then another, told me I was lovely then said he'd walk with me and 'shall we just go through this [really dark] park' and I gathered all my faculties together, told him I was going home and legged it to the nearest tube station - he followed me but my swipe card enabled me to just walk on through the gates, leaving him behind.
I once considered fighting a speeding ticket based on the fact that I passed two atm surveillance cameras right before I got pulled over. My rationale was that if I subpoenaed each bank and reviewed the security tapes, I could figure out the exact time that I passed the Wells Fargo camera and the time I reached BofA, calculate my speed, and prove that I wasn't guilty.
I would have been an utter waste of taxpayer money so I decided not to