A Sea Train (海列車,Umi Ressha?) is a locomotive that traverses on the sea surface. To date, there are only three in existence; two were built by Tom the Fish-Man, a reputed and legendary shipwright from the city of Water 7, and another built by the Galley-La Company. The first one appeared in Chapter 322 and Episode 228.
By design, a sea train is roughly immune to changes in weather. The train's momentum and speed allow it to pulse forward in any situation, even against an Aqua Laguna (a large storm that hits Water 7 every year). Whenever the train passes over the track, a device near the wheels makes an underwater dissonance that drives away Sea Kings. The tracks of a sea train lie just under the surface of the ocean, and sway with the tides to avoid any damage caused by the waves. The tracks require only occasional maintenance, as they are only used to boost the torque of the sea train's paddles on the ocean's surface, as well as a guide to its destination.
Sea Trains were used as transportation by the people living in a small archipelago where the Straw Hats landed. Several sea train lines connect Dock Island and Secon Island as well as other surrounding islands. The rail system was far more elaborate than the one used by Water 7, employing signals and junctions for easier travel. The trains themselves were also more modern and refined, their design drawing inspiration from the 3700 "City" Class locomotives used by the Great Western Railway.
Around 24 years ago, Tom was considering building a proper sea train when he was approached by representatives of the World Government, who sought to arrest the fish-man for the felony of constructing Gol D. Roger's pirate ship, the Oro Jackson. At the time, Water 7's remoteness and proximity to a large number of Sea King made the port city undesirable for trade, slowly eroding the city and all who lived there. By building his "steam paddle boat", Tom would effectively save Water 7 by opening it up for trade, as well as save his own life. The judge overseeing Tom's trial agreed to postpone Tom's crimes for ten years; if he was successful at building the sea train network, he would be pardoned for his original crime.
When Tom began construction on Water 7's sea train network, he had already built a prototype of a steam engine that he called Rocket Man. After drawing up a new blueprint. Tom's polished train, The Puffing Tom, was originally only connected to Enies Lobby, but Tom took an additional four years to build tracks to three other nearby islands: St. Poplar, Pucci, and San Faldo.
The sea train is also responsible for Franky becoming a cyborg. When Spandam boarded Tom onto the sea train, Franky (then Cutty Flam) became enraged and tried to stop it. He did a full-on tackle to try to stop the train. When it ran him over, Franky's injuries were so severe that he had to replace his broken and damaged body parts with steel scrap.