Why is it so Hot in the Metro in Paris in the Summer?

The Answer:
Well… First it’s a common misconception that it’s cooler underground… In a normal underground, let’s say a cave, an underground cellar, etc. It’s not cooler, but at a constant temperature (usually between 50 and 70°F), so it’s cooler in the summer or in South Cal but it’s also warmer in the Winter (this is why caves are a good shelter in Winter, even if Cavemen didn’t actually live in caves, but I’m getting off-topic here)…

So, normally an underground tunnel is at constant temperature, but only if there’s normal underground tunnel activity in it. That is: moss growing, a few creatures dwelling in it, the usual rain infiltration, wine bottles being stored there and stuff like that…
It’s hardly the case in a metro tunnel. One can find trains running around like crazy in there…
So, yeah it’s hot in the metro for two main reasons: the heat generated by the trains, and the one generated by the people…
People underestimate their heating power. One imagine that in a thing as big as a subway system, people are just like ants and not that “important” in it…
Well, just like in an ant pile where ants constitute the majority of the mass and of the generated heat, humans might not constitute the majority of the mass in the subway system (these trains can be pretty heavy), but they definitely generate most of the heat.

Add to this a few factors like no A/C and you end up with a pretty hot subway system all year long, but especially in the summer…

Now, there are differences from metro line to metro line.
Some lines (2 and 6 for example) have big chunks above ground which allows the air to circulate better.
Some lines (8 and 7? Definitely 14 and the brand new trains on line 2) do have an A/C system (or a cooling system anyway).

And then, you have line 4:
Entirely underground, second busiest line in town (after line 1), quite old trains and a unique braking system that generates a lot of heat by itself… And you end up with a monster that might play a significant part in global warming (and in passing outs in Paris in the summer).

I’m not sure about RER B though (I avoid RERs like the plague).