The cart is useless. I've made very long fairly level roads, plus bridges, which takes forever, and the cart can still barely get over it. To get silver down from a mountain, I make a stairway all the way up... and the cart just goes off the path and crashes down randomly somewhere that is hard to recover.

You are best off making a chest every 100 feet. Overload yourself fully - walk to the next chest - unload - wait for stamina to regen - and reload.

It is nice to have a super long flat road with bridges to follow, so it's not a wasted effort. But carts are still useless. Either load yourself to the max, or overload yourself fully; either way will by faster than using a cart.