Payment information, location data and trip patterns can all be attached to our ridership data. Agencies say they use it to better understand how riders use the services and make improvements. But the flip side is transit agencies selling user data to advertisers like a lot of private companies do, or sharing it with law enforcement. We submitted Freedom of Information Act requests to several large police departments across the country — including in New York City, Baltimore and Chicago — for more information on requests they had made to local transit agencies for data over the past decade.