Design a "Public Transit Ticket-system" #36

Detail the architecture required for a mobile app that can serve as a replacement to the Public Transit system of a metro-city, such as NYC subway Metrocard. Please make use of a whiteboard to communicate your architecture and solution.

Requirements --

Your application must do the following

• Allow users access into the Subway without the use of the physical Metrocard
• Allow users to view and manage their account balance
• Charge users for their entry into the Subway

Things to Consider

• Not all Subway stations have Wifi or Cellular network access
• Millions of people take the subway every day