Moreover, if power becomes significant you can add regenerative breaking to regain a large fraction of energy (if they're not doing so already; maybe not because of the age of the trains).
This article is a great read: https://www.ianvisits.co.uk/blog/2017/06/10/cooling-the-tube...
Apparently about half of the heat generated in the tunnels is/was from braking. And the tunnel temperatures have risen from 14-30C since around 1908.
Is a good read about the engineering challenges.