With the growing popularity of contactless payments, the humble Oyster card has a diminished role on the London Underground. Still, it's a useful option for tourists, children, or anyone who wants to keep a close eye on their travel spending. Today, Transport for London (TfL) has launched a mobile app for iOS and Android that makes it easier to top-up and review your balance. Once your account is set up, you can top-up from anywhere — no more queuing at a ticket machine. After 30 minutes, you simply need to touch a reader and the amount will be added on automatically.