I have 17 pieces of 6’8" radius, and 10 pieces of 10’ radius.
All are code 250 NS rail on Micro Engineering narrow gauge ties. All used; no joiners. Each piece is about 2’ long. I’m not absolutely positive on the diameter/radius. ME claims 6’8" for the radius. I laid it out and it seems close - I got about 13’ 2" D, but I didn’t have any joiners on.
I really couldn’t measure the 10’ R stuff as I don’t have enough pieces to get half a circle, but it is definitely larger radius - here the 6’8" radius piece is on top of the 10’ radius piece.
I bought it used in 2005, but never used it, and it’s been in the garage all this time. $80 plus shipping for all 27 pieces!