Darkwood

You must take the musician's path. This means that after you obtain the feces-covered key, you must not show it to Wolfman, but must instead go to the silo and show it to the musician.

Continue on the musician's path. Get his violin from his parents' house in the Old Woods. Find him at the Church ruins and show it to him. After you show him the violin, you will have an appointment with the doctor. The musician will give you an approximate location, but will not mark it on your map. You're looking for "Train Wreck" in the Old Woods. It will be along an edge and there will be a bulge. If you find train tracks along the edge of the Old Woods but don't get "Train Wreck" marked on your map, you're not in the right place; keep looking. You'll get "Train Wreck" marked before you ever see a train or tracks.

Go into the train far enough for the door to close behind you. It's a trap! You will faint from gas, but don't worry, you haven't failed the achievement. The doctor will talk to you and you will wander around in a dream as the doctor. "Doctor" (you) will find "Me" (you) on the floor. Interact with "Me" to become the real you again, but you're still dreaming. Now you need to run. To escape, you will run more or less in the direction you came, but there's some weaving and turning involved. You must hop a fence near the beginning, keep going, pass through a wooden door, keep going, pass through another wooden door, keep going, squeeze past some furniture, keep going, squeeze through a narrow opening, and keep going. As long as you keep moving, you might take damage, but not enough to kill you. If you see that you're about to die, you'd better dashboard out or pause/quit and restart from the dream.

After you escape, you will then wake up from the dream, and the achievement will unlock. Now go see the doctor and decide what to do about your key.