My American husband learned a version from a friend in New York State. It has the lines:

Hitch the wagon to the cart
Go to town to get a load of bart
Some was black and some was blacker
Some was the colour of chawed tobacca

He has always wondered what BART is. I might add that he is a linguist so he has searched around a bit. Any ideas? It can't be bark. You wouldn't go to town for that. And it's big. It comes in loads that you need a cart for. Driving us crazy.