Indeed, I think a few people are missing that MCD4x4 is building a subway layout, not a steam/diesel layout, which means that he doesn't have to worry about pulling long trains, and he doesn't have conventional passenger-layout worries about 80-foot cars on sharp curves.
In fact, it might be worthwhile to build a test track from flextrack to find out what radius these cars can reliably handle. Electric interurban layouts with good trackwork can have curve radii as sharp as 8-10", although that's probably pushing it for an articulated subway set. 15" is pretty darn comfortable for traction and 18" is downright broad, as long as you keep in mind clearance between the tracks.