Each portable payment device associated with a single account within a payment processing system is distinguished using track data. The track data from the portable payment device is read at each of a plurality of merchant point of sale terminals (POS). Rather than relying on the PAN alone, a merchant may utilizes the track data, or a proxy thereof, as the unique identifier for the portable payment device. The merchant may then process transactions involving the portable payment device based on the unique identifier. For example, in the transit environment the transit fare for each rider with different portable payment devices but the same account can be calculated using the unique identifier, such as the full track data read from both tracks of the corresponding portable payment devices.