Amsterdam/Rotterdam

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Amsterdam/Rotterdam
I took a trip to Amsterdam this last week and it baffled me the system for buying tickets on them you buy it from a driver then get it checked by a conductor in a box on the tram who then makes you Tag in like on Oyster in London... Seems a waste of time to me?? far to complex it seems?

Also i noticed a Tram in Feyenoord Livery in Rotterdam anyone know the identity of it? is there more than one as a lot of them carry adverts in this region?