U.Bahn and S.Bahn

http://www.tripadvisor.ca/Travel-g187323-c105026/Berlin:Germany:U.Bahn.And.S.Bahn.html

http://europeforvisitors.com/munich/articles/u-bahn-s-bahn.htm

http://www.gettingaroundgermany.info/stadt.shtml

https://en.wikipedia.org/wiki/Rapid_transit_in_Germany#U-Bahn ,
https://www.youtube.com/watch?v=IdCeRW4dObI ,
https://www.youtube.com/user/UndergroundBerlin

https://en.wikipedia.org/wiki/S-Bahn#Classification

https://en.wikipedia.org/wiki/Urban_rail_transit#Urban_rail_transit_types