I used to do this kind of drawing in the early years of my career. In those days, there was a special pen called a "railroad pen", that was basically two ruling pens on one handle, specially designed for drawing parallel lines. The distance between the 2 nibs was adjustable, and the two together had an optional swivel function, so that you could do freehand curves.
It was a tricky pen to use, if you wanted really good linework, and usually it was necessary to go back and clean up your work after you'd finished. I actually had better results with a single ruling pen. Draw a line, carefully measure the offset and mark with tiny pencil ticks, draw the parallel line, repeat. It just takes patience and diligence. 
Charles Syrett
Map Graphics
http://www.mapgraphics.com
Hi all. This is my first post (excluding my introduction) so if you missed that let me preface this post by mentioning that I draw pen and ink maps by hand as a hobby and I am trying to improve my technical ability in this area.
I've always had bad luck drawing roads (think of any typical USGS topo map road style) because I can never get the two lines to be equidistant from a drawn or imaginary center line. I find that French curves make for unsympathetic transitions and flexible rulers are...flexible. Is there some big secret I'm missing? It seems as simple as a two point nib but I have yet to find one. Am I missing a very obvious secret to accuracy in this area?