Where the series' iconic overworld music opens with a two-note descending fifth motif, which remains intact even in divergent incarnations like Twilight Princess's overworld, Spirit Tracks instead opts for a two-note ascending sixth, which maintains the _feel_ of the original while introducing a wildly different piece. In fact, the descending fifth motif seems conspicuously absent from this piece entirely. In fact, there are very few fifths in this track at all.
#FelthrysVGMSelection
Instead, outside of some chords, a sixth appears to be used wherever one might expect a fifth. I think this gives the music a more open feeling, fitting the intent they had of making the world seem large, ready to be explored.