Serbia-born, Paris-based architect Jug Cerovic moonlights as a map designer and worked to create a guideline called INAT that unifies transit paths around the world.
He redesigned 12 international metro maps using this standard, including the Seoul Metro-Map
I love his design, it improves the usability of the maps a lot.

The official map.

See all 12 maps here:
http://www.inat.fr/metro/seoul/#prettyPhoto