I believe the biggest cause is the heat from braking the cars all day. There is no regenerative braking in the system and the older trains have crude speed control so they're constantly braking to scrub speed between stops for curves and work zones. It all gets dumped as heat into the tunnels.
I believe the biggest cause is the heat from braking the cars all day. There is no regenerative braking in the system and the older trains have crude speed control so they're constantly braking to scrub speed between stops for curves and work zones. It all gets dumped as heat into the tunnels.