Forest Realm
The Forest Realm is the southwestern quadrant of the re-established kingdom of Hyrule and the first region to be explored by Link in Spirit Tracks. As is implied by its name, it encompasses many forested areas throughout the region (much of which are the Lost Woods) and is home to much of Hyrule's Hylian population as it is the home region of Hyrule Castle, Castle Town, and Aboda Village. It is also the location of the Forest Sanctuary, the home of the Lokomo known as Gage, and the Forest Temple, one of the four special temples scattered across Hyrule that serve as the energy sources for the Tower of Spirits and thus serve to sustain the existence of the Spirit Tracks. Link explores the deepest parts of this region after acquiring the Forest Rail Map from the Tower of Spirits and restoring enough of the lost Spirit Tracks to meet with Gage at the Forest Sanctuary.
Spirit Tracks
The Forest Temple that resides deep within the forests of the Forest Realm. The temple becomes tainted when a beetle-like creature known as Stagnox takes up residence in the temple and steals the temple's Force Gem from the temple's energy beacon, thus deactivating the temple's power source and critically weakening the Tower of Spirits. As a result, the Spirit Tracks all across the Forest Realm began to vanish, cutting off the only known road to the tainted temple.
The Forest Realm is the region in which Link begins in Spirit Tracks, as it is home to his dwelling, Aboda Village, where he resides with the ancient Niko and studies under his mentor, Alfonzo. After receiving his certificate to officially become a Royal Engineer in service to Princess Zelda, he learns from the princess in secret that Chancellor Cole is suspected of a plot to remove the Spirit Tracks all across the land in order to release the Demon King Malladus from his prison underground. Though he attempts to smuggle Zelda out of Hyrule Castle to visit the Tower of Spirits in secret to investigate this phenomenon, Zelda and her troupe are ambushed by Cole and his associate, Byrne. The chancellor confirms the princess's suspicions by revealing his true nature and uses dark magic to separate Zelda's spirit from her body before making off with her body, intending to use it as a vessel to revive the Demon King. Link journeys with Zelda's spirit to the Tower of Spirits, where they meet the wise Lokomo sage Anjean, who reveals the truth of what has happened to the Tower of Spirits and the reason for the disappearance of the Spirit Tracks. She instructs the pair to retrieve the Forest Rail Map from the first segment of the tower in order to restore enough of the tracks to reach the Forest Sanctuary, where they can meet with the Lokomo known as Gage and fully restore all of the tracks to the Forest Realm. After recovering the rail map, the pair are given the Spirit Train by Anjean to reach the Lost Woods.
Link navigates the Lost Woods successfully and reaches the Forest Sanctuary, where he restores all of the lost Spirit Tracks by performing in a duet with Gage using the Spirit Flute. With the tracks restored, Link is able to finally reach the Forest Temple deep in the heart of the Lost Woods. There he confronts many forest-related enemies and puzzles, eventually finding the temple's treasure, the Whirlwind. This treasure comes in very handy when he finally reaches the temple's beacon chamber atop the structure. Here he discovers the cause of the disturbance in the temple's energy flow: Stagnox, the Armored Colossus. Though the creature was immense in size, Link eventually discovered its weak spot on its behind and was able to exploit it to finally destroy it. Amongst its remains, the young hero recovered the temple's Force Gem and restored it to the temple's beacon, thereby restoring its source of energy and restoring the flow of energy from the temple to the Tower of Spirits.
Link would continue to visit the Forest Realm a good many times for the remainder of his adventure, visiting Castle Town and Hyrule Castle as well as his home village of Aboda Village at certain points.