I think Manhattan provides a strong counterexample. The 2nd Ave subway line cost $2.5 billion per mile. The cost to add elevators at 70 stations is expected to

I think Manhattan provides a strong counterexample. The 2nd Ave subway line cost $2.5 billion per mile. The cost to add elevators at 70 stations is expected to be $5.5 billion. A lot of signaling equipment is from the 1930s.