Originally Posted by bike2math
I think that is a mod, it looks like an Xtracycle to me. When I was a kid we had these things called banana seats, but I haven't seen them around much anymore, and I'm not sure they can just be added to any old bike anway...
I have a summer project lined up to build a trailer big enough for an adult passenger. But the Xtracycle isn't a bad idea either.
I think those banana seats used a seatpost, and some small poles in the back - mounting to to the frame like a luggage rack. We should see if anyone would put one of these on a modern road or touring bike!