When it comes to rails there is one city in this country that is second to none, Chicago. And where does 66 start? That's right Chicago.
US 66, as well as US 12, US 14, US 41, the Lincoln Highway, and Dixie Highway either start in Chicago or pass through Chicago. This is for a couple of reasons of coarse there are the obvious ones like connecting the large city of Chicago to other large cities like New York city or Los Angeles, and/or Chicago is along the way.
More importantly though there are a few other reasons why Chicago is such a key point along the way. Geographically speaking Chicago was at the time a centerpoint for both maritime, and rail commerce in the area making Road commerce in these area a need as well. Chicago's centrality in the middle of the country as well as in the heart of the Midwest, and its location next to Lake Michigan have given it a key role in collecting and disbursing transportation traffic and materials.
It's Chicago's close proximity to rails though that play a key point in the development of Route 66 as well as other US routes and name routes. In the construction of rails the railways are often forced to level land's so that the rails can be constructed upon them. The railway also keeps lands level next to the tracks in order to have vehicular access to the tracks for repairs and maintenance. These leveled areas stretch on as far as the tracks do. So when it came time for the US government to build highways across country it was a wise choice to look to the railways. Lands next to the railway tracks were flat and level by the railways and also where found in terrain which made passing though mountains easy since the railway wished to have low gradients for their locomotives to climb with little effort. This made road travel for cars and trucks easier to.
Chicago being a central railway point and hub of the nations railway was a point in the country that had rail connections to just about every city in the nation heading East, West, North, or South. Meaning following the tracks from Chicago you could get to any other city you choose. So building roads from this point became a highly logical concept. In 1926 when 66 and its sister routes where first constructed, Chicago became a focal point exactly for these reasons.