A nice discussion at 37 signals about the use of abstraction vs. realism on subway maps, looking in particular at maps of London's (which is highly abstract) and New York's (which is more realistic) standard subway maps.Helpful distortion at NYC & London subway maps
It is not obvious that one is inherently better than the other for all purposes. As noted earlier on the post about Could you walk it quicker, the London map imposes huge distortions. On the other hand, the more complicated the map, the more that is required to be filtered, arguing for more rather than less abstraction.
The specialist publisher Capital Transport has put out a number of books about the history of the Underground, and its maps, which are quite interesting.