Japan was the first country to build the high speed railway. In 1964, the first high speed railway line, Tōkaidō Shinkansen was opened, connecting Tokyo and Shin-Ōsaka with the high speed trains 0 series with up to 210km/h. The 0 series is the first high speed train operated in Japan. It had put into service for more than 40 years, and was once the symbol of Japan.
Why do you think Japan needs to built high speed railways to connect the big cities?
Jasper's Blog on Trains 
I think Japan wants/needs to connect the larger citys by rail because it is probably the fastest and most cost-efficient way of travel.
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japan is a country that relies on each other for resources. It is a country that is very productive and needs fast access to its resources thus the high speed railroad was built.