It really is deeper than the others you list — only Barcelona comes at all close. The other issue is it has a few lines that are really crowded, which means wha

It really is deeper than the others you list — only Barcelona comes at all close. The other issue is it has a few lines that are really crowded, which means what cooling ability it has is pushed to its limits. The fact the subsurface lines have had many of their historic vents closed off for above-ground development now steam trains are a thing of the past doesn't help there, either. At least Glasgow has other environmental factors keeping it cool — the fact it's a short circle with two underground crossings of the Clyde (which keeps the surrounding tunnels cool) really helps it in the summer, as well as moderating the temperature in the winter. It does get pretty ridiculously humid in the winter compared with outdoors — I have many memories of walking into the stations just to have my glasses steam up and be unable to see anything! It's also much, much shallower than the deep level lines in London (the deepest station in Glasgow is something like half the depth of the deepest in London).