Trying to get to the Forest Temple will probably not work, because there's something weird about the tracks there. You'll get lost and end up going in circles, and will have to give up.
Instead, take your train to the village in the center of the map. Be sure to use your train whistle to make the cows and monsters leave your train alone. The monsters won't go away right away, but just keep blowing the whistle until they go away.
This village is Whittleton. There is a shield in the shop here if you didn't get one before and want one now. Go to the big hut in the back of the village to talk to the Whittleton Chief. He tells you to get help from the villagers for figuring out the correct path through the woods. One of the villagers will tell you to pay attention to the trees, but ignore the fourth tree.
So, get back on board the Spirit Train. Go to the west, into the forest. Now, remember what the person said about listening to the trees. What that person meant was that the tree branches will be pointing in the direction that you should turn your train when you get to a crossing. But! Don't go where the fourth tree says to go, because it has a bad sense of direction. Go in the opposite direction of where the fourth tree is pointing.
If you went the right way, you will reach the Forest Sanctuary. Watch out for crows outside. If they hit you, you will drop some rupees, which they will steal from you unless you kill them before they fly away. Go to the northeast part of this screen to find some stairs leading up. You will find bomb flowers here, so take one back to the rocks that were blocking a stairway back the way that you came. Blow up those rocks, but go back the way that you came from.
Go past those bomb flowers that you used, then cross the bridge to the small hill where there's a switch. This switch will make a temporary bridge appear, so what you need to do is place a bomb flower next to the switch, then go get another bomb flower. When the bridge appears, use the bomb that you are carrying to break the blocks on the other side of the temporary bridge. Now you can continue to the next area.
In this area, there is a statue shaped like a rocket ship that is making some noise. Tap on it and you will see a closer view, as well as some colorful lights floating out of it. Take note of those colors, then use your Spirit Flute to play the song that those colors are indicating. This song is the Song of Awakening. It wakes up things that are sleeping. Playing the song will make a statue appear, who tells you that you can wake statues up if you play the Song of Awakening near them.
Go to the left and check on the picture in the center of the circular area. It says to connect the statues that are facing each other. So go around and look at the statues. Make notes on your map of which ones are facing each other (draw an arrow over each statue to indicate the direction that it is looking in). Then go back to the picture in the center and draw a line between the two statues that you discovered were facing each other.
The door will open and you can go inside. There you will meet Gage. Talk to him and Gage will say that you need to play the Spirit Flute to play a song with him. You will practice first, then after you are done practicing, he will have you play a different song. When you have played the song correctly, more Spirit Tracks will appear on the Forest Map. Get back on board the train and go to the Forest Temple.