I think the proposal is to tunnel under the Japan Alps from the outskirts of the Kanto (where Tokyo is) to the outskirts of the Kansai and Osaka. The tunnel is

I think the proposal is to tunnel under the Japan Alps from the outskirts of the Kanto (where Tokyo is) to the outskirts of the Kansai and Osaka. The tunnel is mostly required because Japan's mountainous geography is not friendly to straight corridors, but it means fewer birds and sticks will be a benefit.