I think they're referring to cooling of the train cars.
The deeper tube cars running on older lines have serious space constraints to deal with that make integration of things like cooling equipment difficult and expensive. We're talking about trains that have comically small proportions relative to what you would see anywhere else in the world.
That said, last I had read, the deep line cars are supposed to get AC sometime within the next decade or so.
>I think they're referring to cooling of the train cars.
Don't think so. That is a fixable problem. (e.g. convert the space of 1 seat per car for an AC unit).
The fact that everyone is so stumped by the problem says it's the actual tunnels that are overheating. The fact that some lines don't have AC just exasperates this.
The deeper tube cars running on older lines have serious space constraints to deal with that make integration of things like cooling equipment difficult and expensive. We're talking about trains that have comically small proportions relative to what you would see anywhere else in the world.
That said, last I had read, the deep line cars are supposed to get AC sometime within the next decade or so.