The layout has to live in a small space (ideally 12"x32"). The list of gauges which can make a 180-degree turn in 1' is really short. Z gauge can do it with Marklin 8510 (5 3/4" radius) track. I can probably get a locomotive that'll handle that turn, but ... Z gauge is really small. Really really small. Putting any electronics of my own design inside a Z gauge car is likely to be really difficult. Add to that the fact that Z gauge is really expensive. Locomotives and cars seem to be twice or more expensive than their N gauge counterparts. Finally, the Z gauge world is also small. There's Marklin, MTL (Micro Trains), and AZL (American Z Lines). Oh, and Marklin just declared bankruptcy. I had an Amiga once. I don't need another.
N gauge is bigger. Bigger minimum radii (well, except for a Japan-only manufacturer named Tomix which sells 4" curves), more manufacturers (Walthers lists 2,564 locomotives in N gauge versus 144 for Z gauge), and lower prices. It's also the second-most-popular gauge in the world, next to HO. But again, the problem is with the turning radius. The smallest radius sold is the Kato Unitrack 216mm (8 9/16"). It's unfortunate (we end up with a layout that's 18"x40"), but bearable.
I don't want to use the Kato Unitrack for the real design. First of all, it commits me to designs which can be implemented using their hardware. Second, the track has integrated ballast, which always looks weird. Assuming that Atlas Flextrack can accomodate the turn radii, it should be sufficient. Unfortunately, 8 1/2" isn't the tightest turn on the layout. The turns from the switches to the crossing are sharper. I still need to find precisely the right position (possibly the angle too) for the approaches to the crossing, but I know of at least one solution which gives greater than a 6" turning radius on all four corners.
The trick now is to find a locomotive and cars which can handle a 6" curve. An initial investigation suggested that the Kato EMD NW2 was the way to go, but then I found another page suggesting that the NW2 wasn't very good at all when it came to extreme radii. That second page said the Atlas GP-35 had no trouble with 6.4". Hopefully it'll work for me too.