Yeah it's called proof-of-payment and it's starting to become more common in the US. In San Francisco all of Muni is POP, it speeds up boarding dramatically.

Yeah it's called proof-of-payment and it's starting to become more common in the US. In San Francisco all of Muni is POP, it speeds up boarding dramatically. As for electronic ticketing, dunno how much it's caught on in Germany but BVG in Berlin has a mobile app that you can buy tickets with. Worked well enough for me.