Jim, a lot of pet dogs are trained to drag sleds with weights--there are competitions at some local dog shows for this. My favorite was a little terrier (of course the weight of the sled is in proportion to the dog's size and weight) who came through obedience classes with my Hysson. She was a really enthusiastic puller! I know a lot of pet dogs who pull carts or sleds, too.

There's a big difference between pulling on a weight-pulling harness and pulling on the leash fastened to the collar, and any dog can be trained to tell the difference!