With the Spirits' Train, you will be able to ride to your destination. Furthermore, you can now draw your desired path with the stylus and the train will automatically head that way. However, you can always manually go a different way. You can also tap Route in the bottom-left corner to redraw the entire route.
Before heading to Gage, you should go to the area on the map that looks like a town south of Hyrule Castle Town. It will show up as ??? when you select it while drawing the route.
While on the way, be sure to blow the horn if any Moinks get on the tracks. You will also have to fight Snurgles, which float in front of the train and then bash into it after a while. Blow the horn repeatedly until they go away.
When you get to the town, you learn that it is Whittleton. When you want to leave, tap Zelda to get back on the train.
You can get a shield now if you didn't get one.
Go into the big house in the back of town and talk to the chief. He'll tell you that you have to go a certain way through the woods or you'll just end up where you started. Go talk to the people in town to find out how to get through the woods.
Go into the house to the right of the chief's house. The villager will tell you to pay attention to the trees, but ignore the fourth tree.
There's an area to the northeast in Whittleton that you can explore, but you can't really do anything there right now except kill some Spinuts. Just go to the train and make your way to the Lost Woods, which are to the west.
In the Lost Woods, you will eventually see a dead tree at the side of the tracks just before the rail switch appears at the bottom of the screen. A branch of the dead tree will point right or left. Make the rail switch go in that direction. However, be sure to go the opposite way from where the fourth tree is pointing, because that's the one that has the "bad sense of direction" like the villager said.
You can't control the speed of the train in this area, so be sure to switch directions as soon as you know which way to go.
After you go past the fourth tree, you will exit the forest and automatically stop at the Forest Sanctuary.
To find Gage, first go north from the station, then go right, up a couple of short flights of stairs. Take a bomb flower and run back down the two flights of stairs, then keep walking to the left and throw the bomb at the stairs that have some blocks at the top.
Next, go back to the bomb flowers. There is a switch across the bridge to the left. That switch makes a temporary bridge appear to the left. There are some blocks just past that temporary bridge that you will need to blow up.
You will need to use a bomb to create the temporary bridge, then take another bomb across the bridge to blow those blocks up.
Get a bomb flower and put it next to the switch, but don't throw the bomb directly at the switch, because it will activate the switch immediately, not giving you enough time to get another bomb flower and run across the bridge with it. When you put the bomb in place, wait for a moment and then go back and get another bomb flower and run across the temporary bridge and throw the bomb at the blocks to blow them up. It might take a few tries to time it correctly.
Once you go past where those blocks were, you will notice a rocket ship. Tap on it, and some colored lights will float out of it while making some music. If you tap the Spirit Flute, you can play it. Just drag the flute so that the note that you want to play (i.e. blue, orange, yellow, etc.) is in the middle of the screen under the blue line, and then blow into the microphone to play that note. Play the notes that the rocket ship is playing (blue, orange,) and Link will learn a song for the Spirit Flute: the Song of Awakening. You can use it to wake up sleeping statues like the one that you just woke up, and anything else that is sleeping.
Continuing on, there are some stone statues around a sign. Read the sign and it says "connect the statues that are facing each other." Go around and look at the statues to see which two are facing each other, then check on the sign again and draw a line between the two statues that are facing each other. When you do, the door will open. Go inside.
Go north up the stairs and you will meet Gage. You will do a duet with Gage. After the automatic scenes, talk to Gage and he will teach you how to play the Spirit Flute. Blow into the microphone when you're in control of Link. Then Gage will have you practice the song. The timing is important, so be sure to practice.
After that, press the arrow at the bottom-left of the screen to stop practicing. Talk to Gage when you are ready to play the song for real. He won't play the same notes as he played during the practice session. When he is done playing his part, he will turn to you, and you have to play your part. Then he will play some more, and you have to play your part again. Then you will need to do that one more time. When you have done it correctly, more Spirit Tracks will appear.
If you didn't learn the song from the rocket ship, go ahead and do that now. Then you can leave the Forest Sanctuary and get in the train and go to the Forest Temple. Be sure to blow your horn to make the enemies along the way leave you alone. And don't worry about the signs along the track. Nothing bad will happen if you ignore them.