新宿駅

Shinjuku-eki is the world's busiest train station. Four railway companies—JR, Odakyu, Seibu, and Keio—serve the station, and three of the Tokyo Subway's lines also stop here.

The entire rail hub at Shinjuku handles anywhere between 1.5 million and 4 million people each day, depending on who you ask. According to Shinjuku Ward's web site, the average daily passenger stats look like this:

JR              1,490,306
Keio              699,395
Odakyu            491,862
Tokyo Metro       255,994
Toei Subway:
  Shinjuku Line   231,421
  Oedo Line        88,336
Seibu             203,988
TOTAL ......... 3,461,302

That's a lot of guys in suits. If you're looking to get crammed into a commuter train like a sardine, Shinjuku is definitely the best place on earth to do it. Go sometime between 7 and 9 AM for best results.

Why all these people? The population of the ward itself is a paltry 270,000, and although Shinjuku is a large commercial center (and the seat of the Tokyo metropolitan government), it's nowhere near as massive as Ginza, Nihombashi, and the financial district east of the Imperial Palace, around Tokyo Station.

Well, to understand why Shinjuku is so busy, you have to understand its role in the Tokyo transportation grid. Of the 30+ million people in the metropolitan area, most live south or west of the city center. Shinjuku is the westernmost point on the Yamanote Line, which encircles Tokyo and brings many of its commuters to work. Therefore, most of these three and a half million people are only coming to Shinjuku to change trains.

And so many trains! In addition to thousands upon thousands of local trains, Shinjuku also handles several limited expresses: the Narita Express to New Tokyo International Airport, and the JR Hakone and Azusa trains that connect Tokyo to points west. The Shinkansen, however, doesn't go to Shinjuku. Yet.

Some good photo sites:

http://www.3deearts.com/tokyo/shinjukueki.html
http://www.virtualtourist.com/m/tt/99a4/
http://www.webcom.com/whills/Japan/Shinjuku.html