My threshold object comes from the game  Riven.

The track structure is rather simple.  The thing is, though, that getting out of it and onto the exits is the tricky part.
You can go a lot of places by using the submarine, and there it can come out of the water through a funnel-shaped hole.  You can look up, open the hatch and attempt to step out. The problem is that unless you  had put down the ladders you cannot go out. To get the ladders down you have to solve a certain puzzle in the game.
 
 

This sub is pretty cool becuase it functions as it hsould and by simply clicking your mouse on levers you move around the virtual place. The surroundings change according to your new position and you may even get disoriented. The  cool graphics and accompanying sounds aid the object's function in drawing the player into its reality. The picture ceases to be an image and becomes a funcitonal submarine.  The cool thing about it is that it lets you do anything you would want it to. By putting it on tracks they eliminated the player's urge to try and move it up and realize that the programmers do not want him to do that. So this way you can only stop in the designated places, but it is not because they did not make more places, but because it cannot move off the tracks.  If its functions are limited, they are limited by its "direct" physical surroundings, and are therefore readily accepted.
The whole idea of functional objects reminds me of a drawing I saw once of a pipe. It said "Ceci n'est pas un pipe" (this is not a pipe). This is meant to remind us that pictures are merely  2-D combinations of lines and colors. In this case, this is a sub, especially afeter a few minutes of  being inside of it.