I came up with the concept for this track about two years ago, but I have nowhere near enough room anywhere in my house to implement it. Hopefully some day we'll move to a house where I can make it a reality. Even after all that time, it's still the layout I dream of.
I have plans somewhere that I drew out on graph paper, but they've been swallowed up by my home's black hole. Here's a quick drawing from MS Paint of what I hope to build some day:

I can't recall the dimensions, but you're probably looking at about at least a 24' x 8' footprint, with a bit longer area at the base of the downhill run on the left side that I did not account for in the drawing. It should extend out another couple of feet to allow for a further drop in elevation. The optional oval lanes would be enacted by use of mechanical flipper switches. Basically, attach the flippers to a rod that sticks out of the benchwork at the front of the track. Yanking the rod would move the flippers and enable the use of the lanes.