I think I read that the warmth from people and braking has baked some of the clay earth around the tunnels, making them even more insulative!
Because of how old they are there is some really interesting long term data as the earth itself around the tunnels reaches a new thermal equilibrium. In looking for a source, i found a good wikipedia article on tube temp in general.
These comments and the article made me see how much of it is/was due to braking. TIL.