Most subways are heavy rail, not light rail. And, as you note, underground subways rarely go away--particularly after the age of multiple competing lines, often

Most subways are heavy rail, not light rail. And, as you note, underground subways rarely go away--particularly after the age of multiple competing lines, often redundant. All save one of the defunct lines in NYC went defunct because they were demolished for other stuff--because they were elevated or at-grade lines. (The one underground line that went defunct was split into three lines in active use today.) Putting it underground definitely increases political and budget survivability.