Excellent point! The Japanese started engineering for this as well when they went through a bonanza of submerging rail lines in Tokyo. At first, they started d

Excellent point! The Japanese started engineering for this as well when they went through a bonanza of submerging rail lines in Tokyo. At first, they started digging super deep with smaller tunnels (the trains use a linear motor/reaction-rail for much tighter loading gauges). However, they started to realize that it made the the now submerged trains more inefficient to be so deep. More recent lines are elevated or much more shallow.