Go to the wall that looks like a big V with a flat bottom.
Hold A and run up and down the V.
If done right you will appear on top of the wall.
Press B. You should now be able to walk on the wall.
If done on the right side of the train when you spawn, you can walk to the right when on the wall.
If you go down until you look down and see land. Try to roll down next to the pole.
Now enjoy Exploring! Remember though bring an exploding weapon to kill yourself if you don't want to be there the whole game.