The reason the northern line (on the tube) is so hot is due to 2decade+ of heat building up in the surrounding rock. Not the same dynamics because of convection

The reason the northern line (on the tube) is so hot is due to 2decade+ of heat building up in the surrounding rock. Not the same dynamics because of convection in water, etc, but an interesting and very real example of how local buildup of temperature can be hard to manage over multiple years.