Boarding times though? I agree of course about the cool factor and I was sure to ride one when visiting London, but afaik the slower (dis)embarking times are why double-decked trains are primarily used on intercity trains and not on commuter/local trains or light rail (metro etc.). Typical busses stopping every two minutes are probably similar

Edit: wait, I know one in Germany as well. It's expensive but between Frankfurt airport and another major city, there is a rail line but somehow the bus is significantly faster and they're using double deckers. The front row at the top was, n=3, always free for me (all four seats) so you get a quite unique experience, at least so far as my bus experiences in mainland western Europe go :)

How about articulated double-decker buses? The one does not need to exclude the other after all. It'd need a large harmonica to deal with hills and bumps or it could have separated top floors with sloping roofs.