Another update:
Starting the reverse Loop!!
I soldered together a full piece of flex to a cut-off piece to make a 51" long flex. I bent the flex using 18R RIbbon-Rail gauges (I have both the 5" and 10" in that radius and LOVE THEM!!). Then I tried another experiment.
Instead of trying to lay out and draw an 18" radius curve, I simply put the bent flex to the desired radius (18"), placed and pinned it a few places, double-checked the radius of the bend. That is one nice thing about Micro Engineering flex - it stays bent rather than spring back to straight. Now it does have a little spring action, so those ribbon-rail gauges come in VERY handy. After confident of placement, I took a Sharpie and put a dot in the approx midpoint between every 3rd or 4th tie or so. Unpinned the flex and used that 10 inch long 18R ribbon as a guide to connect the dots and draw the centerline of the curve. Yes, I know that the outside edge of the ribbon rail is actually a tad more than 18" radius, but it was close enough.
Split pieces of WS foam roadbed and glued to the drawn centerline. Pinned and let dry overnight. Next morning I checked the bent flex to the roadbed and am very happy with the result!
Glued down the flex with caulk after using one insulated and one metal joiner to connect to the TO (the PSX-AR I'm using as an auto-reverser says to offset the 'breaks' by 3/4 inch. I'll go back and use a cut-off wheel to cut a gap and fill with a piece of styrene). Weighed down with ready-at-hand weights (really, not bought special for this) - my now-trademark bottles d'Vino. Use Bordeaux style bottles, not Burgundy. Bordeaux are the straight bottles, and Burgundy (or Pinot Noir in the States) have a larger curved shoulder. The straight give a better weight-to-contact distribution.
One other ??trick?? Having read that the soldered joint of a curved flex is the most likely place for a kink to occur, I kept the 5-inch 18R ribbon rail in place over the soldered joint while the glue (latex caulk) dried. This keeps everything nicely in gauge and true curve.
Checked last night and I am quite pleased with the results!
Thanks for the read, Steve J