There’s also ventilation, and pumping if you’re going to be digging below the water table. These are ongoing costs: you need to keep incurring them for as long as you want your tunnel system to remain viable, not just during the initial digging phase.
This page suggests that avoiding water during the mining process is yet another complicated and surprisingly expensive topic, and one that often requires exploratory digging before one commits to a major tunnel. I don’t know if transit systems handle it in the same way, but it’s worth noting that people tend to build cities at low elevations and near major bodies of water.
There’s also ventilation, and pumping if you’re going to be digging below the water table. These are ongoing costs: you need to keep incurring them for as long as you want your tunnel system to remain viable, not just during the initial digging phase.
This page suggests that avoiding water during the mining process is yet another complicated and surprisingly expensive topic, and one that often requires exploratory digging before one commits to a major tunnel. I don’t know if transit systems handle it in the same way, but it’s worth noting that people tend to build cities at low elevations and near major bodies of water.