A friend of mine competes with her Newfoundlands and they love it. I did competetive weight pulling with a previous gsd and she loved to pull, I was thinking of doing carting with my boy now as it seems almost too easy to teach but the startup costs are high. To buy a proper cinch harness and then make/buy a cart, I haven't been bothered.
You can order a nice weight pulling or xback harness though and they can pull a sled or cart with that but the steering is somewhat dangerous. Much better with a proper cart and harness.
Google draft dog. Most of the stuff you find will be about Newfies or berners but there is some good info out there.
Eli - Hellwigg's Freedom in Stride RN RA SCRB1 CGN TEC
Cajun - Hellwigg`s Hot for Karma CGN TT UCC
Evie - border collie mutt CGN
Sweetie - APBT CGN