Checking in by phone for public transport – you need to know this

With a smartphone you hardly need a wallet anymore. Flight tickets no longer need to be printed, but can be scanned directly at the airport via your phone. And paying is a lot easier now that you can place your phone on the payment device. It seems only a matter of time before you can also use your smartphone to check in on public transport. But how long do we have to wait? And what can you expect?