The Heavy Transport is an improved version of the Light Transport in Age of Empires. This upgrade doubles the carrying capacity and adds +50 additional hit points as well as slightly increasing its movement speed. Depending on the type of map, the upgrade may not be needed in most games, but if played on sea maps with many island chains, it is strongly recommended. The Heavy Transport is available to all civilizations.
"Ships built for carrying military units replaced smaller galleys when armies grew larger and targets became more valuable and better defended. It became necessary to move ever-larger armies for invasion, and to bring siege engines and supplies along for extended sieges of coastal cities. The Heavy Transport represents a larger sailing ship, something like a Merchant Ship, built mainly for capacity at the expense of speed."