With the Spirit Tracks now opened in the Forest Realm that will take us to the Dark Realm, let's get the train rolling again in that direction. You'll notice now that the Dark Trains are gone from the Forest Realm tracks, but instead we have pirate tanks rolling along pegging shots at us. (If you restart the game from the last save point being the Tower Of Spirits, though, the Dark Trains will reappear.) Blast them with your cannon to keep them off your back. A portal will appear at the end of the track that will take us to our destination.
The Dark Realm itself is a maze of tracks, all patrolled by Dark Trains, and there are portals that will transport us to other connecting tracks in this realm. Here our goal is to destroy all the Dark Trains before we get destroyed. Along the tracks there are Tears of Light which will give the Spirit Train energy to destroy the Dark Trains for a short while -- sort of like Pac-Man with trains. While empowered, blowing the train whistle will give our train a burst of speed. So carefully plot and replot your course around the tracks so that you can get the Tears of Light before you collide into the Dark Trains and then drive your train straight through a Dark Train to destroy it. Once all the Dark Trains in this realm are destroyed, you'll move on to your next target: the Demon Train.
The big black locomotive engine with the evil face finally makes its appearance along four parallel rail tracks we'll need to share with him. During the course of the battle, the Demon Train will take up two tracks and will at times shift over toward the tracks we're riding on. When that happens, slow down so that he will go in front of us, then switch over to another track and speed up again to keep up with him. Also, watch the bar on the right side of the upper screen, because this indicates how much track you have left to travel on to complete that part of the battle (with Link as a diamond going up on the bar). When you run out of track, Link's train falls off and you must start that part of the battle over.
In the first part of the battle, we need to take out five sections of the back compartment that are filled with explosives before they are dumped off the train onto us. Just wait for those sections to open up in order to blast them with the cannon. When they are all destroyed, the back compartment will be decoupled, and the Demon Train will go through a portal. Follow the train through the same portal.
In the second part, we need to take out laser cannons that will pop out of the remaining car and fire at us. These cannons take two hits to defeat, but there are only four of them. Also watch out for flaming barrels that appear on the tracks. Once that car is decoupled, the Demon Train will go through another portal.
In the third part, the Demon Train itself will fire rotating lasers from the sides of the train. Blast at the crystals firing the lasers at either side of the train when they start flashing, which will cause it to slow down drastically once they are all destroyed. At that point, speed up your train so you can get in front of the Demon Train and fire at his face. Keep repeating this until the train finally comes to a halt, totally defeated.