The book Transit Maps of the World by Mark Ovenden gives a splendid overview of how metro/subway maps evolved to chuck the twists and turns of roads and rivers and instead just show a schematic of how stations relate to one another.  The straight lines and 45-degree angles of transit maps have become standard in metros from Moscow, to London, to Atlanta, and even some bus system maps (here). 

Not-to-scale maps are really the best way of showing where transit goes without cluttering up the image with how it precisely gets there.  The one deficit, of course is when a passenger wants to gauge the distance between lines/route, for example, to avoid a transfer.  Or if you want to show a colleague the distance between BART stations.

BART System Map

The BART system is interesting in that it behaves like commuter rail outside the CBDs of Oakland and San Francisco, but is more like light rail with stops a half mile apart nearer the urban core, or even a bus with only 3/10ths of a mile between Embarcadero and Montgomery Stations.  But you wouldn’t know it from looking at the BART system map; there is some distance between Castro Valley and West Dublin, but it doesn’t look like 10.2 miles. 

Well, one enterprising individual with an unpronounceable website name has gone to the trouble of producing scale version of the BART map.  You can see it here with several other iterations of the Bay area system map.