After fighting Byrne, you must go to the Sand Sanctuary.

Go to the Sanctuary

Go to the Ocean Realm from the Tower of Spirits and ride the train into the enclosed area in the desert. This is the Sand Sanctuary. Read the sign here and wake up the two statues. You will learn that you need to bomb the area where the gazes of the big statues meet. The sign is referring to the big statues that you saw while riding around the rails. One of the statues says that the layout of the Sand Sanctuary is the same as the layout of the rails.

You need to go out and figure out which direction the large statues are looking in, draw it on your map, then go back to the Sand Sanctuary and bomb the spot that the statues are looking at.

The place that you should bomb is south of the Sand Sanctuary's spot on the "map" and is to the left and down from the first intersection south of the Sand Sanctuary's spot. When you find it, go downstairs.

Inside, you meet Rael. Perform the duet with him to add new Spirit Tracks.

To reach the Sand Temple, you first have to go to the Fire Realm using the northbound rails to the east.

Rocktite

You will go through a tunnel and there will be a Rocktite in there that will attack you. Fight it like you did before. This time, the Rocktite will sometimes not have its mouth open. Hit one of the barrel bombs on the wall when the Rocktite walks over it to make the Rocktite open its mouth, then tap its eye like crazy to hit it.

Twisting Tunnels

The tunnels here warp you around to different points. Make a note of where the tunnels send you so you can make it out to the path going back south to the Ocean Realm. The direction that you go through the tunnels changes where you go out, so be sure to try going through each tunnel both ways. You might need to go into reverse as soon as you go through a tunnel in order to go the right way.

After going through the arch into the Twisting Tunnels, the correct way through is to go in reverse as soon as you emerge from the first tunnel, go in reverse again after the second tunnel, then turn right. You'll come out on the correct track.

Turrets

The third and final trial is that you have to kill all of the cannon turrets that are mounted on the Sand Temple. Just hit their cannonballs with yours, then shoot them once to kill them. Then the door to the Sand Temple will open and you can go in.