Much of the cost of the subway is the access caverns, life safety systems, etc. For example, NYC built a 2-mile water tunnel for $250M [1]. A subway tunnel cost

Much of the cost of the subway is the access caverns, life safety systems, etc. For example, NYC built a 2-mile water tunnel for $250M [1]. A subway tunnel costs in the ballpark of $500M-$3B per mile[2]. [1] https://jalopnik.com/meet-pat-the-drill-thatll-dig-a-new-underwater-tunnel-5903263 https://jalopnik.com/meet-pat-the-drill-thatll-dig-a-new-und... [2] https://sf.curbed.com/2018/6/18/17464616/bay-area-subway-train-rail-costs-price-bart-muni https://sf.curbed.com/2018/6/18/17464616/bay-area-subway-tra...