Tondemo nai, Yamanote sen desu
(What do you mean? This is the Yamanote line!)
The Yamanote Line, operated by JR, is Tokyo's best-known surface railway line, tracing an oblong ring around the center of the metropolis through most of the city's major railway stations. It looks like this:
(6)
\
Komagome------Tabata (7)
/ \ / .---(8)
(5) Sugamo Nishi-nippori / /
| / \ / /
(4)----. | Otsuka Nippori------(9)
\ | / |
(3)-----Ikebukuro Uguisudani
| |
(2)--. Mejiro Ueno (10)
\ | | /
Takadanobaba Okachimachi /
| | /
(1)----. Shin-Okubo ,-----------Akihabara------(11)
\ | / \ |
(25)------Shinjuku (1) '-Kanda .-----(12)
/ | / | /
(24)---' Yoyogi-----------' Tokyo--------(13)
| |
Harajuku Yurakucho
| |
(23)-----Shibuya Shinbashi------(14)
/ \ |
(22)----/ Ebisu Hamamatsucho
/ \ / |
(21) Meguro Tamachi \
/ \ | (15)
/ '---Gotanda Shinagawa
(20) / \ / | | \
/ Osaki | | \
(19) | | | (16)
(18) (17) (6)
The other lines in this map are:
1 - JR Chuo Line to Nakano, Hachioji, Tachikawa, Takao
2 - Seibu Shinjuku Line to Nakai (follows Yamanote Line from Shinjuku to Takadanobaba)
3 - Seibu Ikebukuro Line to Tokorozawa, Irima
4 - Tobu Tojo Line to Kawagoe
5 - JR Saikyo Line to Urawa, Omiya (follows Yamanote Line from Ikebukuro to Osaki)
6 - JR Keihin-Tohoku Line north to Urawa and Omiya, south to Kawasaki and Yokohama (follows Yamanote Line between Tabata and Shinagawa)
7 - JR Tohoku Shinkansen to Sendai and Morioka; Joetsu Shinkansen to Niigata; Hokuriku Shinkansen to Nagano (follows Yamanote Line to Ueno and Tokyo)
8 - Keisei Line to Narita Airport (follows Yamanote Line to Ueno)
9 - JR Joban Line to Matsudo, Kashiwa, Mito
10 - Tsukuba Express to Tsukuba (will eventually extend to Tokyo)
11 - JR Sobu Line to Chiba, Narita Airport
12 - JR Sobu Rapid Line to Chiba, Narita Airport
13 - JR Keiyo Line to Urayasu, Chiba; Musashino Line to northeast Tokyo
14 - Yurikamome to Odaiba
15 - Tokyo Monorail to Haneda Airport
16 - Keikyu Line to Haneda Airport, Yokohama, Yokosuka
17 - JR Tokaido Shinkansen to Odawara, Nagoya, Kyoto, Osaka (follows Yamanote Line to Tokyo)
18 - Rinkai Line to Odaiba, Kiba
19 - Tokyu Ikegami Line to Nagahara
20 - Tokyu Meguro Line to Denenchofu
21 - Tokyu Toyoko Line to Jiyugaoka
22 - Tokyu Denentoshi Line to Futako-Tamagawa and Nagatsuta
23 - Keio Inogashira Line to Kichijoji
24 - Odakyu Odawara Line to Atsugi, Odawara, Hakone
25 - Keio Line to Fuchu, Hachioji, Takao
There are also a number of Tokyo Metro and Toei Subway lines that intersect the Yamanote, but I have left these out for readability's sake.
"Yamanote" or "Yamate," both literally meaning "hand of the mountain," are used in many Japanese cities to refer to the hillier outskirts of town, in contrast with downtown (called "shitamachi"). The Yamanote Line originated as the Shinagawa Line, a linear run from Shinagawa to Akabane (north of Tabata on today's Keihin-Tohoku Line) which opened in 1885. The section from Tabata to Ikebukuro, originally called the Toshima Line, was opened in 1903, and the Yamanote Line was formed by merging and electrifying the two lines in 1909. At that point, the north side of the line was still relatively underdeveloped in contrast to the Ueno-Shinagawa corridor, so the Yamanote Line really did connect Tokyo's yamanote to its shitamachi.
The loop was completed in 1925. Nowadays, It takes just over an hour to circumnavigate the line, whether on a "sotomawari" (outer-loop, i.e. clockwise) train or an "uchimawari" (inner-loop, i.e. anticlockwise) train. Trains usually exit the loop for servicing through a connecting line south of Osaki, and enter on the Saikyo Line through Ikebukuro.
The Yamanote Line, being so prominent in the local rail network, is also extremely crowded at times, particularly during the morning rush hours. Most trains on the line now have cars with folding bench seats, which are stowed during rush hours to make the car standing room only.