Looking at the Chuo Shinkansen, it seems that the tunnel allowed the train to turn less through the mountains, considering that turning is pretty hard to do at 500 kph! Also:

>JR Central aims to begin commercial service between Tokyo and Nagoya in 2027, with the Nagoya–Osaka section originally planned to be completed by 2045.[5] The government is however planning to support a speed up of the timeline for the construction of the Osaka section by up to 8 years to 2037 with additional funds.[6]

Imagine a US government construction project being finished ahead of schedule.