Fot those of you interested in the actual radii and length of the USA Track LLC rails, here is a list:
42, 57, 72 and 87 curve tracks result in concentric circles that are on exact 7-1/4" centers.)
STD42 (2.143 gauge, 2.268 c to c) 45º-8/circle
arc=15.112, radius=19.241, diameter=38.482
arc=16.002, radius=20.375, diameter=40.750
arc=16.893, radius=21.509, diameter=43.018
STD57 (2.134 gauge,2.259 c to c) 30º-12/circle
arc=13.873, radius=26.496, diameter=52.991
arc=14.464, radius=27.625, diameter=55.250
arc=15.056, radius=28.754, diameter=57.509
STD72 (2.125 gauge, 2.250 c to c) 22.5º-16/circle
arc=13.254, radius=33.750, diameter=67.500
arc=13.695, radius=34.875, diameter=69.750
arc=14.137, radius=36.000, diameter=72.000
STD84 (2.125 gauge, 2.250 c to c) 22.5º-16/circle
arc=15.683, radius=39.938 diameter=79.875
arc=16.125, radius=41.062 diameter=82.125
arc=16.567, radius=42.188, diameter=84.375
STD87 (2.125 gauge, 2.250 c to c)22.5º-16/circle
arc=16.101. radius=41.000, diameter=82.000
arc=16.542, radius=42.125, diameter=84.250
arc=16.984, radius=43.250, diameter=86.500