What is the name of the first high-speed rail line fully financed by a private company?

The line identified with the first privately financed high-speed railway project is Taiwan High Speed Rail.

Taiwan’s north–south high-speed railway was developed through a build–operate–transfer, or BOT, concession. Taiwan High Speed Rail Corporation was given responsibility for financing, constructing and operating the system before transferring it back to the government at the end of the concession period. This model made the project unusual among major high-speed railways, which were generally planned and financed directly by state railway organizations.

The project began as a government-planned railway, but legislation in 1994 opened the way for private infrastructure finance. THSRC won the concession in 1997 and signed the BOT agreement with the government in July 1998. Construction officially began in March 1999.

The line uses Japanese Shinkansen technology, despite an earlier bid based on European TGV and ICE technology. Limited commercial service began on 5 January 2007, and full service between Taipei and Kaohsiung followed in May. The phrase “fully financed” describes the intended private-finance BOT structure; in practice, the project later received substantial government financial support.