My cargo bike is a homebuilt Frankenbike designed and built specifically for hauling cargo. See my Avatar. 'The idea was that a smaller rear wheel would lower the center of gravity, and improve overall stability. Anyway, one might call it a class of bikes with many different makes and models.

I tend to use trailers with regular road bikes most of the time, and only drag out the cargo bike out when I need to carry something on the larger trailer, so really it doesn't get a lot of use carrying stuff. However, I've had over 100 pounds on the rack, and about 500 pounds on the trailer.

Mine was built around a 20x4.5 rear wheel. Kind of a fun idea, but in the end not so really practical. However, the rear derailleur tends to overhang the rim a bit when shifted to the lowest gears which causes problems with the small, wide rear wheel.

Suntour also made a triple pulley derailleur that pops up for sale from time to time. The idea is to wrap a little extra chain with a shorter derailleur cage. When stretched forward, chain wraps the rear two pulleys. When relaxed, the chain wraps all 3.