There has been a series of curved pieces of metal that are intended for
setting the curve radius of flex track on the market for a long time. They
do work to a degree but do tend to be rather erratic in their action as far
as I'm concerned, especially in the longer radius version. If you're doing
something like 15" radius, I'd suspect that they'll work just fine.
My preference when doing flex track would be to draw a set of lines forming
the inside and outside tie lines and bend the track so that if fits inside
the lines. This works well for the track that will hold it's shape. For
the floppy track, just draw a line delinating the edge of the ties on the
roadbed and glue the track down to the line.
Another choice, especially with larger radius curves is to make up a large
plywood piece so that it butts up against either the rail or the ties and
lay the track down with that.
Why do penguins walk so far to get to their nesting grounds?