The article is not quite explicit about that, but it does say "cooler running trains could help to cool the tunnels, but it would take decades to notice the difference, or they can use that temperature gap between the new and old trains to put air conditioning inside the carriages today... If a future plan to run even more trains through the tunnels goes ahead, that will require increased cooling in the stations, which, funding permitting, is being developed at the moment."

The implication seems to be that an equilibrium (or something very close to it) has been reached for the current traffic volume (and putting aside climate change, presumably.)