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[quote user="artschlosser"]Railroads cannot use a trammel and circle center to lay out an arc or a curved section of track.  Rivers, mountains, hills prevent this. But a table can be prepared to use a simple method of using a straight line and measuring offsets from this straight line.  The table has offsets for the various curvatures that are required.Some 60 years ago while in high school, I came across a book that described this method. 1. At the start of the curve, Point A, extend a line 100 feet to Point B.2. Consult the table for the offset needed for the desired curvature; at Point B, measure the offset at right angles from the line and establish Point C.3. Sight a straight line from Point A to Point C and extend the line 100 feet to Point D.4. At Point D, using the same offset from <a href="http://www.serverlogic3.com/lm/rtl3.asp?si=31&k=step%202">step 2</a>, establish Point E.5. As <a href="http://www.serverlogic3.com/lm/rtl3.asp?si=31&k=in%20step">in step</a> 3, sight a straight line from Point C to Point E and extend the line another 100 feet to establish another point.This process is continued until the straight line is headed in the proper direction (called a tangent) that can be extended to the start of the next curve. Flags are positioned at points, A, C, E, etc., and the track crew grades the right-of-way accordingly.The table that I found had offsets specified for the mid points of the extended lines but I doubt that they were really necessary although they would define the curve more accurately. Art [/quote] Hello Art, That is interesting. Interesting enough for our techie to check how other railways do it. We have a book from the RhB (Rhaetian Railways) which is strictly technical info on aspects of track geometry, various tables and formula. The RhB uses 3 different "Cords" (10m; 15m; 20m), in addition to the main offset (1/2 "Cord") they also state the offset at 1/4 "Cord". Interestingly no mention is made of curvature expressed in degrees.  OTOH 100ft converts to 30.48m, which poses the question: was a smaller "Cord" used for designing and laying out NG railways with tighter curves?Interesting! ER  PS RhB uses the 1:x for turnout angles. [;)]
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