The game comes with a route editor, which is where you can build your own routes. It is very tempremental and is full of quirks. Almost every single person who tries to build a route highly recommends buying the bible of rotue building: "A Guide to Route Building" by Michael Vone, published by Abacus. It comes as a downloadable 340+ page .pdf document, available for US $ 20.- at http://www.abacuspub.com/.
I have it and it is a step by step guide on how to build a route. It explains everything in a step by step method. You can either just read the parts on how to do stuff to build a route, which are in blue boxes, or you can read the how and why you do it that way in the rest of the book.