Upgrading it is slow and expensive work, but the new systems will eventually allow more trains to run on the same track.
The New York subway is old. You see the signs every time you ride it. The paint on the cast iron pillars is layered like the rings of a tree trunk, and if you removed the ads from the walls, the stations would fool any time-traveler from the 1950s into thinking they were still in their own time.
