[soruly]

and yes they did. aside from the keihin touhoku line that's already running in parallel with yamanote line, JR East added a new track called ueno-tokyo line about 10 years ago. And the trains on the new lines are mostly 15-cars (with 2 double decker cars!). Note that for passengers travelling north-south they can also take the west route (going through shinjuku instead of tokyo), and there're also 3 parallel lines on the west. JR runs on narrow gauge which takes less space, and is cheaper to construct. Maybe this is one reason to make these expansion project fesible. https://en.wikipedia.org/wiki/Ueno%E2%80%93Tokyo_Line