When you go to the area where the bridge is supposed to be, you will discover that the bridge is broken. Maybe someone can fix it for you. Someone who works on bridges, maybe.
Before you deal with the broken bridge, stop at the train platform just before the bridge.
This is the Trading Post. Go into the house and approach the counter to meet Linebeck III. He'll ask you to go to the Snow Realm and go bring the Bridge Worker to fix the bridge. Agree to do it.
Bridge Worker's Home
To get the Bridge Worker's Home, go to the Snow Realm, then follow the tracks that go south of Ferrus's house.You will pass a train platform. This is the Bridge Worker's Home. Go inside and ask the guy in the house to fix the bridge. He'll go to your train and wait for you there.
When you get back to the Train Platform, Ferrus will be there. He'll tell you that the Bridge Worker is strict about the train, so Ferrus says to obey the signs for blowing the train whistle and changing speed, to not get hit by enemies, to stop right at the train platform, and to not use the emergency brake to brake suddenly.
When you're ready, get on the train and go to the Trading Post, where you met Linebeck a short while ago, making sure to obey the rules like Ferrus said. If a sign says to change speed or blow the train whistle, do it just before you reach the sign. Be sure to switch to the slower speed before getting too close to the train platform so you can come to a smooth stop without having to use the emergency brake. And make sure not to go past the end of the train platform at your destination.
Revisit the Trading Post
Back at the trading post, the bridge worker will get to work. Go over to the bridge by walking east from the train platform, and talk to the bridge worker there. Linebeck will claim that you agreed to pay the fee of 5000 rupees.
Go and talk to Linebeck and he will say that he has an idea. Agree to do the work. He'll tell you about the Regal Ring hidden nearby, which is worth 8000 rupees. Agree to look for it. He'll give you Linebeck's Letter.
Find the Regal Ring
Go west from Linebeck's house. The gate that was locked is open now. Go inside.
Go north, but be careful of the Like-Likes walking around. They will pop up and try to eat you and take your shield, so try to avoid getting eaten. If you want to keep your distance, just throw two bombs at it to kill it.
Near the stairs at the north end of the room, the pot on the right is shaking. There is a Like-Like hiding in it, so be careful. Jump to the left and use a bomb on the rock to gain access to the stamp station. Then go up the stairs.
Outside, you will find Linebeck's grave. There is also a rocket statue here. Check on it to learn the Song of Light. A light beacon will emerge from the ground and shine a beam to the northeast.
Linebeck's letter says "sound the light and follow its beam." Take out the boomerang and draw a path to the northeast where the light is shining. You will find a crystal switch over there, so hit it. A bridge will appear. Cross it, then use a bomb flower to open the crack in the wall. Go in.
There are a bunch of octoroks around, as well as a fake treasure chest in the north part of the room that actually contains a Like-Like. Start by getting rid of all of these enemies.
Linebeck's letter says "Inside, go 4 steps north and 6 steps west from where the lights cross." Play the Song of Light next to the two light beacons in the room. Walk 4 steps north and 6 steps west from where the lights cross. You don't have to stand precisely on the spot. Play the Song of Discovery when you get there. A chest will appear that contains the Regal Ring.
Go back to Linebeck in the trading post and tell him you found the Regal Ring. The bridge worker will overhear and take the ring from Linebeck. Then Linebeck will ask you to bring him treasures in exchange for new train cars. Using different train cars can increase the number of hearts that your train has.
Now go back to the train platform. Talk to the bridge worker on the way and he will warn you to cross the bridge quickly because it's shaky in some places. By the way, if you lost your shield in the cave, the cheapest one can be purchased from Hyrule Castle Town.
Go across the bridge and into the Ocean Realm. Once there, go past the first train platform. It's Papuchia Village, but there isn't much of anything that you can do there right now. Instead ,go past and stop at the train platform on the small island.
Go northwest, using your boomerang to stun the crab from behind and walk past. Go into the cave entrance in the northwest area.
Kill the crab in your way and go around the room. Pick up a bomb flower and throw it over to the blocks around the crystal switch. Then use the boomerang to activate it and extend the bridge. Walk across, kill the crab, and go up the stairs.
Go to the left and find the statue. Make a note on your map of which way the statue is looking. Then go north. There will be a barrage of rocks that some octoroks are spitting from the left. Use your boomerang to activate a crystal switch on the island to the left, then cross the bridge and kill the octoroks. Make a note of where this statue is looking. Then go back across the bridge and north, and make a note for the statue up here.
Then go back to the south and this time walk to the right, going past a red door. Go to the southeast and make a note for this statue, then keep going southeast until you get back to the screen with the train platform. There is a switch here that extends a bridge, giving you a shortcut so you don't have to go through the cave again.
Go back to the north. Make a note for the state in the northeast corner of this area, then go up the stairs to the right. There are some chickens here. Pick one up and glide south to get to the island on the east side of the screen. Step on the switch to extend a bridge, then go up the stairs and make a note of where the statue is looking. If there is a chicken with you on the island, pick it up and glide south to find a chest containing a random treasure.
Go back to the door and draw the pattern that you drew on your map by following the gazes of the statues. Go inside.
Carben is not here, but he left a note saying that he is in Papuchia Village, so go back to your train and go back to Papuchia Village, which you passed on the way to the Ocean Sanctuary. Be sure to make a shortcut by going southeast from the sanctuary and pressing the switch in the screen below.
Walk over to the center of the village until you see some birds flying around. Zelda will notice Carben flying around with the birds, but she can't get his attention. Zelda recommends asking the Wise One for help.
You don't actually have to talk to the Wise One, but if you want to, she's in the biggest hut in town, which has a fish above the door. You will need to talk into the microphone and she will say something about a secret near the lonely tree where the birds fly.
Go to the palm tree under the spot where the birds are flying and play the Song of Discovery. A rocket statue will appear, playing the notes Purple-Yellow-Purple. Play this on your flute to learn the Song of Birds. Carben will come down to ground level and talk to you. He'll agree to help and will ask you to take him back to the Ocean Sanctuary in the train.
Get in the train and head toward the Ocean Sanctuary. Pirates will attack before you get there. You'll be in the passenger car, and Carben will ask you to protect him from the pirates. Just keep an eye on the upper screen to see where the next pirate is about to come in, then go and kill it. More and more will show up, so just stand near Carben and kill the pirates as they come over.
Eventually, a giant pirate monster guy will show up. It's a Big Blin. Hit it over and over, but run out of the way if it swings its club back. Wait for it to swing the club. It might swing the club twice, so wait for it to be holding the club on its left side before you go back and start hitting it again. If any miniblins go over and attack Carben, get rid of them and then continue attacking the Big Blin until you kill it.
Once it's gone, set a course for the Ocean Sanctuary. Make sure to stop the train before the platform and don't use the emergency brake at high speed.
When you arrive at the Ocean Sanctuary, Carben will give you a Force Gem, which will reveal some new tracks on the Forest Rail Map. Then Carben will go to the sanctuary. Go to the sanctuary and perform the song with him. A bunch of new tracks will appear, and Carben will give you a letter explaining how to find the entrance to the Ocean Temple.