To answer the original question - leaving aside the very specialised (and currently unobtainable) "barrel circle" tracks mentioned above - the tightest radius "production" track I'm aware of is Bertram Heyn's "Radius 0", which is, as you'd expect from the name, smaller than standard Radius 1. It is available with either standard style sleepers or some rather attractive "random" rough-hewn narrow gauge type sleepers that look nice for small industrial or estate lines. You can either use them with standard R1 points, or Heyn do some very nice (though quite expensive) Rad 0 points in either built-up or kit form.
I have a circle of the Rad 0, and have done some tests with LGB locos and stock - the small Feldbahn locos and wagons handle it just fine, as does the standard LGB 4-wheel power block, though it is VERY tight for the regular-length LGB 4-wheel stock. A Stainz will run round it happily with some feldbahn wagons in tow, if run bunker-first; running smokebox-first is just about possible but right up against the limits of coupler lock due to the longer rear overhang of the loco.
I would say that the R0 track is useful stuff for specialised uses (such as a minimum-space industrial layout where you still want a continuous run loop) but I certainly wouldn't use it for a more general layout.
Jon.
