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Flyer Track has about a 19 inch radius (at track center)
Flyer 'Pikemaster' has a 15 inch radius
On my 'high rail' layout  I use 19 to 25 inch track radii. 
Life is filled with compromises. (Paul)

Comments on track radius 'Standards' from Ted Larson  mhrreastNOSPAM@juno.com

Bill Krause, Tom Hawley, Don DeWitt, and other NASG members researched the radius question when developing module standards for S scale in the early '80's.  Bill reported on this research in the club newsletter, and in the NASG Dispatch.

Their objective was to find curves that would look good, permit use of large steam engines and passenger cars, and be small enough to transport.

The following information is from Bill's report. "NMRA will tell you that curves should be no less than 54 inches." Doug Miller used an engineering graphic computer to develop a design using inside curve of 46 inches radius, and outside curve with a radius of 47-9/16 inches.  Additional work by Bill using NMRA and John Armstrong date showed that using a track separation of 3-3/8 inch center-to-center would "allow a Big Boy locomotive to pass, on the inside curve, and 85 foot passenger car traveling on the outside curve."

These standards, now known as the S-MOD standards, are equally applicable for all forms of standard gauge S scale (AF, hi-rail, and model) railroads.  My club uses code 148 rail and we run all variations of S scale, standard gauge.  Some clubs also have incorporated narrow gauge on their modules.  The standards are available through the NASG; $8, plus $5 for a corner module addendum.  There is a discounted price for NASG members (everybody in S scale is an NASG member, right???); $5, plus $3 for the addendum. Ordering information is in most issues of the NASG Dispatch.  I currently fill the orders, so you may contact me directly.

Ted Larson