Originally Posted by
evergrn
Google Map shows 4 tracks running between Shinagawa and Osaki. Maybe they won't need to build additional tracks for this purpose.
Yep, that's the Yamanote Freight Line. Those tracks merge with the Yokosuka Line on the Shinagawa end, and the Saikyo/Shonan-Shinjuku Line on the Osaki end. This is the route that the Narita Express takes between Shinagawa and Shibuya, but AFAIK JR doesn't run any other passenger trains on it.