
This new game directly follows the previous one. It's a hundred years later, and a new kingdom of Hyrule has been founded on the continent that the protagonists of Phantom Hourglass discovered sometime between that game and this one. Wait a minute, that would make Link and Zelda more than a century old, and--hmm, are we just assuming they're immortal at this point? Is there any sort of rationale at work in this timeline anymore?
You know, who cares? It's a Zelda game. The indigenous people of this new land tell of an ancient, legendary battle between the good spirits and the forces of the Demon King Maladus, whose soul was finally imprisoned in a centralized Tower of Spirits. Maladus is down but not completely out; the ethereal shackles that bind him are in fact stretched far and wide across the new land of Hyrule.
And those bonds just happen to be... train tracks! Convenient, since a train replaces Phantom Hourglass' ship as your primary mode of transportation in Spirit Tracks. The game opens with Link training to become an engineer, during which time he realizes that these train tracks are beginning to disappear. There's also a shady chancellor named Cole involved who may in fact be trying to revive Maladus (there's always one bad apple in these games), so Link starts to explore the land to find out what's going on.

I got to mess around a bit in the second dungeon of the game, which was--brace yourself--the Snow Temple! Yes, themed dungeons are back. I had an item here that let me unleash a targeted whirlwind that could stun enemies, flip some switches, and even propel me around when I was floating around on a block in pools of water. You'll play through these dungeons solo, but you'll be joined by a controllable phantom when you're playing the Tower of Spirits levels. (I wrote about playing with that guy back at E3.)
According to Nintendo, there will be plenty of things to do on the overworld in between dungeons. You can only hop off and run around at stations and towns, but there will be side activities and quests to undertake both at these larger population centers, and you'll run into some interesting characters as you ride the tracks as well.

Phantom Hourglass had a neat multiplayer mode that was sort of like Pac-Man, where one player controlled Link and the other directed some Phantoms around (like ghosts) to try to corner and take Link out. The multiplayer in Spirit Tracks is more of a four-player free-for-all where everyone is scrambling to pick up little Triforce gems and hang onto them till the end of the round. But roving Phantoms will chase you if they see you, unless you can get another player into their line of site. And other players can throw bombs at you or hit switches to drop you through trapdoors, as well as pick up some devilish power-ups to take most or all of your gems from you. The multiplayer seems like a neat diversion, and I'm glad it's not just a rehash of the mode from the last game.
The classic Zelda adventure is the thing that you ought to care about in Spirit Tracks, though. It's really just an iterative update to the core gameplay, controls, and graphics from Phantom Hourglass, but I have no problem at all with Nintendo iterating on a game that great.