I am building a longtail cargo bicycle mostly to be used for around-town hauling but the concept is very common is touring (aka Surly's Big Dummy).
I built this bicycle from a 26" mtb frame up front, top tubes cannabilized from another 26" mtb and then a 20" kids mtb for the rear triangle. The iteration pictured is currently single-speed (for testing).
Future work includes designing and building a rack, installing an internally-geared rear hub, and adding disc brakes. Also, reinforcing material needs to be added to the seat tube.