Zelda Spirit Tracks has Link, a lowly train engineer, tasked with restoring the kingdom's disappearing railroad tracks. The ghost of Princess Zelda is your sidekick, who hopes to be reunited with her demon-possessed body.
The main quest consists of:
Drive train to new temple (sometimes you have to do a side quest to get there)
Beat temple to unlock next floor of the Spirit Tower. Drive back to tower.
Beat new floor by using Zelda to control Phantoms. Unlock new tracks.
The temples and tower floors all had enjoyable puzzles offered new experiences new to Zelda. Some classic items return, and the new items were creative and fun to use. Thank goodness you dont have to repeat the same temple over and over like you did in Phantom Hourglass.
Driving the train is much better than the boat of Phantom Hourglass, IMO. Instead of wandering around a vast ocean, you almost always know exactly where to go and how to get there. You still have to actually ride the train there (which kinda feels like Pac-Man), but usually you get there quick enough to not be frustrating. Also the wild-west-ish music is catchy.
My biggest gripe is that the gameplay is a little too linear. There are a few side-quests, but most of them seems to involve collecting stuff and shuttling cargo around the map. Being restricted to railroad tracks doesn't leave much for exploration. All the towns are very small with only a few buildings and people. Maybe a linear adventure is better suited for a portable system, but hopefully the Zelda series is not eventually reduced to just a collection of dungeons.
Bottom line: Zelda:ST improves on Zelda:PH in every way. It's worth playing just to see the charming interactions between Link and Zelda.