Trains are the major transportation system in Japan. Multiple trains are run by different companies like JR East, Tokyo Metro and private railways. Transferring between different operators often requires passing through separate ticket gates. This is why train stations are always busy and packed with people. The trains in Tokyo for example are estimated to serve around 2.5 to 3.0 million passengers on a daily basis. This is why the stations also serve as food hubs where you can grab a quick meal. Riding the trains and exploring the stations is one of the adventures tourists can do while travelling in Japan.
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From @logen9f