Some of us bend brass flexi track with with nothing more than our bare hands, following a template. I originally started drawing pencil lines on the patio using a wooden lath as a trammel, a nail at one end fitted into a joint in the paving and a pencil at the other end in one of two holes drilled for the inner and outer radius. The lines smudged, people and pets got in the way, it rained, so I came up with the idea of drawing and plotting full size guide lines on A1 with marks to join sheets together. I know not everyone uses Cad and has access to a large format plotter at work, but this really works, bending at short intervals and comparing the results with the lines on the paper to make accurate curves. I found molgrips help to bend the bits close to the ends, avoiding the 'Threepenny bit' or 50 pence piece flats near joints.
Final adjustment of the angle at the end of a curve and forming transition curves can be done on site. I have built up a library of paper sheets with different radii which can be used again to make progress with the current track laying project when its too wet or dark outside. Its basically the same method suggested by Korm Kormsen but without a fixed fulcrum point to lever against, I use that method for taking kinks out of trodden on, mainly second hand track bent in the other direction, its much harder to bend rail in the vertical direction. We have a coffee table with elephants head carvings at each corner and very useful looped trunks, usually closer to hand than the Black and Decker workmate.
Geoff the garden navvie