I purchased one of the game carts that is advertised from time to time. I went to a blacksmith and had him attach a collar to the cart that could in turn be fastened to any bicycle on the vertical portion under the seat. This gives a hauling capacity of a couple of hundred pounds. I don't know how to post pictures here, but if some one wants one, just shoot me an email address.
I have seen everything probably possible as I used to eat at a lunch counter on the main north - south route along the California coast.
One guy had two carts in tandem behind his bike. He travels up and down the coast on a permanent basis. Of course some people load the bikes up up with baskets, luggage carriers with expandable straps or haul baby strollers behind.
One guy my age has been riding around on his bicycle for over 37 years collecting bottles and cans for turn in.