Denen-Toshi Line
The Denen-Toshi Line (田園都市線, den'en toshi sen) is the southwestwards extension of the Tokyo Metro Hanzomon Line and is operated by Tokyu. Originally a tram line, it was converted to a full-scale underground line as part of a cruel experiment to see just how many commuters would pack themselves into a string of wheeled subterranean cans and bears the ignomonious distinction of being the most crowded non-JR line after the Tokyo Metro Tozai Line.
The Denen-Toshi Line runs from Chuo Rinkan way out in the sticks of Kanagawa southwest of Tokyo through to Shibuya, where it metamorphoses into the aforementioned Hanzomon Line with barely a blink, and will, if you stay on long enough, carry you through Tokyo all the way to somewhere ridiculously far north like the Kamchatka Peninsula.