Photo used unfortunately without permission from the late Sheldon Brown. Aaron Goss sent me this link, with the following explanation:
I was the first to put an Xtracycle on a tallbike. I used my old Colnago because the frame was damaged. On the bottom is a Trek a customer gave me. The bikes are mechanically connected. We welded special brackets, but the frames are intact. Sheldon Brown took the picture from his wheelchair. He had a hard time walking towards the end. It was taken at Interbike 2006.
My friend, Kipchoge, co-founder of Xtracycle, was so inspired, he built one also. Actually he built 2. His is fun to ride, but mine is taller!
An Xtracycle is a frame attachment for a standard bike that converts it into a cargo bike. It's like turning a car into a pickup truck.
With this attachment, an Xtracycle-equipped bike can carry groceries, passengers or just about anything.
Watch this video to learn more