"Interestingly enough, track 12 does not only contain information pertaining to the last use, but also to the use before that."
This might explain why the system sometimes asks you to "Swipe again" when going through the turnstiles. maybe the write failed?
In the 90s the system had tokens. Someone figured out a cheap way to game the system using inexpensive South American coins..
https://news.google.com/newspapers?nid=1955&dat=19951206&id=...