It's in a cave:

At the end of chapter 2 where you stepped through the small nitch and got swaped.to the opposite side of the map.
you soon find a circular track that leads to a large door. you must open this door.

remember where you had to back the rail car into a blocked cave in order to go up a long track to a door to open and rocks in there to destroy.

then drive the cart back to Farleys house and along the short track and drive through the wall.and end up on the opposite side of the map.drive along the track and around the circular track and back through the wall.

drive forward a short way to farelys house and back out to the first switch, then back forward to the blocked entrance with red boulders. destroy them and walk into the cave to the airplane, phonograph, and recipes for various drinks.

i wonder how many more secret places there are!