The Poses, Log and Eternal, and Navigation on the Grand Line
Introduction
In the world of One Piece there are five seas, namely the four Blues and the final Grand Line. You might think at the onset of the story that the G.L. is a place of horrors and terror, and that's true for one member of the crew in particular.
Normal compasses are capable of assisting navigation in any of the Blues, but in the Grand Line everything goes out the window. Inasmuch as the many isles of G.L. carry differing amounts of minerals, every island has developed its own unique magnetism over the years. These multiple magnetic fields pretty much blow every compass away in confusion, so a different form of navigational tool is used: The Poses.
Log Pose
The Log Pose is a method of navigating the Grand Line by memorizing magnetic fields. This takes from a few hours to years. The problem here is that there are seven differing paths through G.L., depending on your course from Reverse Mountain. Each has its own twists and turns, so you may go down one path and find a prehistoric island; or you might go down another and find a higher-level island. What all these paths break down to doesn't matter, however, because they all lead to the same location of "Raftel," the fabled island that only Gold Roger and his crew saw firsthand.
The Log looks like a simple spherical bracelet, complete with a compass needle in its center, and is the primary method of navigation for maneuvering the Grand Line.
Eternal Pose
Sometimes you can't wait a week or a month or a year for the Log Pose to set, and that is why the Eternal Poses exist. Rather than memorizing magnetic fields like the Logs do, the Eternal models lock onto one island for all of its existence. In effect, you're trading variability for surity which, to the chaotic-wrought inhabitants of the G.L., must seem like a fair enough trade.
It's been verified that the Umi-Ressha (Sea Train) Puffing Tom carries Eternal Poses to each of its destinations.
Umi-Ressha
With the deadly varieties of the Log and the limits of the Eternal, with pirates and Sea Kings rolling up and down the waves, the people of the Grand Line are cut off from each other in many cases. Entire islands are sliced away from the culture of G.L. and, in the doing, form their own societies and lives apart from the rest of the world. However, for five specific islands, the legendary "Umi-Ressha" (literally, Sea Train) rides the waves to link them.
The Umi-Ressha was the brainchild of Tom, the Fishman shipwright who served as mentor to Water Seven's greatest craftsmen. Because the Great Age of Pirates had further obliterated ties between countries, Water Seven was dying a slow death; sure, it had the craftsmen to build ships, but lacked supplies! That is why Tom devised the Umi-Ressha, which effectively saved the island... but that isn't important here. The islands connected are Water Seven, St. Poplar, San Faldo, Pucci, and Enies Lobby.
Incidentally, in the building of the Umi-Ressha Puffing Tom, its builders made a second model named Rocketman, one that was wildly uncontrollable and dangerous.