Transport for London tracks individuals . . .
A small amount of data about the commuter holding the card, including a unique ID number, is stored on it.
When the card is presented at a tube station or on a bus, the ID number, together with information including the location and time of the transaction, is sent from the card reader to a central database.
In time, Transport for London have a database with the exact movements of a significant number of the people who live or work in London.
Washington DC's Metro has similar "SmarTrip" cards. No indication on tracking. It would be pretty ironic if Metro could have a functional passenger tracking system, when it seems to have a hard time keeping a handle on its trains.
Recommended Reading: The Transparent Society, David Brin
We have not even begun to deal with the real question: "who will watch the Watchers?"
Any ideas about how to find out what Metro does with the data it gathers?