research: http://mic-ro.com/metro/metrorings.html
yamanote loop line - https://en.wikipedia.org/wiki/Yamanote_Line
osaka loop line - https://en.wikipedia.org/wiki/Osaka_Loop_Line / https://www.osakastation.com/the-osaka-loop-line/
meijo loop line - https://en.wikipedia.org/wiki/Meij%C5%8D_Line
(toei oedo line is apparently not a true loop line...)