TheArrivaPerson wrote
Maybe the whole second layer thing is good. Are spirals any good?
Just be sure to have enough risers (the ones that engines can pass under), ascending curved track (essentially a standard curve that is elevated by a molded-on gradual riser), and gradual risers to balance out the descent. Put enough of them all together and you can create some nice layouts using a considerably smaller amount of space with a larger amount of track (curves no less).

Such as, of course, the ever famous Amtrak Super Spiral set, which from my understanding would be able to just fit in the space you told us about.