Your trains will look better and long rolling stock will track more reliably if you add an easement between the straight and curved track. An easement is a gradual transition, or a very broad non-concentric curve, inserted between the tangent track and the circular curve.
Obviously, you can’t have a true easement with sectional track, although if possible you might want to insert a broader radius curve, like a 22″-radius section, between the straight track and the 18″-radius sections. Whether you’re using flextrack or handlaying your track you’ll find easements will keep things running smoother and looking better.
The prototype uses all kinds of fancy formulas to figure out easements, and you can spend a lot of time adapting those same formulas for model railroad use. But that’s hardly necessary. I plot easements using nothing more complex than a pencil and a flexible piece of wood.
Start by drawing the center line of the tangent track directly on the subroadbed. To ensure a constant radius curve use a trammel, which is nothing more than a piece of wood (a 1 x 1 works fine) with a hole for a screw or nail at one end and holes big enough to clear a pencil every inch starting with your minimum radius and ending up with the largest practical radius. Or you can use a template made from styrene, Masonite, or cardstock, cut to the desired curve.
Draw a circular curve of the desired radius. Don’t connect this line to the center line for the tangent track – instead leave a slight offset, about 1/2″ to 3/4″, between the two lines. The offset and length of the easement varies with the curve radius. For an 18″-radius curve the offset is 3/8″, with a 12″-long easement. The offset is 1/2″ for a 30″- radius curve, with an 18″-long easement. Mark the point where the tangent is square with the radius and equally divide the length of the easement on both sides of that mark.
Now you’re ready to mark the center line of the easement. Use a piece of flexible wood molding. Hold the wood along the center line of the tangent and bend it to match the radius of the curve you already marked. For very large curves you may need to temporarily drive a few small brads along the molding to hold it in place. Once you’re satisfied with the alignment trace along the molding with a pencil, creating a nice smooth transition. Remove the molding and lay your roadbed or track in place along the center lines.
This is a simple way to lay curved track with an easement. A detailed explanation of easements, complete with a table showing suggested measurements for various curves, can be found in John Armstrong’s Track Planning for Realistic Operation (Kalmbach). John also presents convincing arguments for using easements.
Marty McGuirk is a former associate editor of Model Railroader magazine. This article was first published in the November 2001 issue.