In the mid 2000's The Legend of Zelda was in the midst of an identity crisis. After twenty years of steady success, the landmark series had become a victim of its own longevity, as the need to innovate clashed up against the expectations of an aging fanbase. Wind Waker was declared too cartoonish and suffered poor sales compared to its predecessors. 'The Legend of Zelda is not for children!', cried men who had played Legend of Zelda as children. 'I demand a serious mature game, to reflect my serious mature life!'
The solution was a split. While the so-called 'Adult' timeline continued on the GameCube and later the Wii, the cell-shaded, kid friendly world of Wind Waker was transplanted to handheld consoles. The first of these games was a fun if narratively confused ride that suffered from sluggish pacing, but the second - Spirit Tracks, or Whistle of the Wide World in Japanese - was exceptional.
Gameplay-wise, the main gimmick of Spirit Tracks is the train. This reportedly raised a few eyebrows at Nintendo during development. Aren't trains a bit... industrial? What's next, Link on an airliner?