London's system is very old so basically it's loud because it's built with much tighter curves than a modern system would be built with today, plus there are few platform doors isolating noise away from people on the oldest lines.
Oh, I see. That sounds like the Oedo line here in Tokyo: it has several very tight curves, so it's extremely loud inside when the train goes around those curves.
But that's not really what I'm talking about with Montreal: in Montreal, the subway system is loud for the people standing inside the station. (The trains are noisy inside too, but that's just in addition.) Every time a train enters or leaves the station, it's deafeningly loud, because of the rubber ties. You'd think it shouldn't be this way (after all, cars have rubber tires and are very quiet at low speeds; it's only at freeway speeds that rubber tires get loud on cars and trucks), but it is. It seems to be caused by the trackways: the tires are constantly rubbing on the sides of the track, so the cars are very noisy any time they're in motion, and it's really miserable just standing on the platform.
