AnyRail can create (near) perfect arcs, easements and straights from flex track.

To create a straight, curve or easement

Alternatively, click the flex, and select from the Ribbon:
 

Creating a precise curve (circular arc) is similar to straightening a piece of track.

An easement provides a smooth transition between a straight and a curve. Real railways always use them to avoid wear and tear, and to permit higher speeds. They also increase passenger comfort.

Page url: http://www.anyrail.com/help_en/index.html?flex.htm