“Most cities with mass transit systems now rely on some sort of electronic card to pay fares. Each of these cards has a unique serial number as well as a magnetized strip upon which its fare balance and expiration date are stored. When you swipe your card to get into the subway, information about when and where you swiped it is automatically recorded in a centralized transit database. And if you purchased your card with a credit or debit card, it will be traceable back to you. Access to transit databases is very carefully controlled, but police have used them in the past to investigate crime suspects by determining whether or not they could have been at the crime scene.”