He is renowned for funding the construction of Japan's first subway system, now known as the Tokyo Metro Ginza Line, which opened in 1927.

[2] Hayakawa studied law at Waseda University and made the bold move of sending an article he wrote about national policy to the statesman Shinpei Goto, who had been serving as head of South Manchuria Railway (Mantetsu), leading to Goto offered him a job as his secretary and starting Hayakawa's career in the railway industry.

[2] In 1914, Hayakawa embarked on a tour of Europe and North America to study rail and port infrastructure, during which he was struck by London's underground railways.

The challenges were myriad, ranging from soft and groundwater-saturated water, lack of subway experience among local engineers and the astronomical cost of ¥6.2 million (approximately ¥3.7 billion as of 2017).

For six months, in rain and shine, he walked the streets of Asakusa, Ginza and Shimbashi and took note of the passing pedestrians, rickshaws, trams, cars and trucks.

The left pocket soon bulged, and through this one-man survey Hayakawa was able to get a rough idea of where people were congregating in the metropolis.

[2] On 27 September 1925, construction finally began on a 2.2-km tunnel running between Ueno and Asakusa, which was referred to by the name Kaminarimon, the iconic gate of Sensoji Temple.

[3] A key rival to Hayakawa was Keita Goto of Tokyu Corporation, who sought for greater dominance over Tokyo's railways.

Hayakawa wanted to build his train line through Toranomon, circle through the imperial palace and back to Tokyo Station, like the mirrored letter “C”.