Prior to the invention of the railroad, all bulk goods (including food and non-precious metals like tin) could essentially only be traded long distance via ship. It simply isn't practical to move them via animal-drawn carts; the animals cost too much, they eat too much, it takes too long, and the roads weren't there.

Rome, for example, was supplied by an unbelievable number of ships' worth of food every day. It would have starved without that ocean access; there's not enough nearby land to be used as farmland to grow all the necessary food in the pre-railroad era.