The main question at the end hasn't been answered yet.
I didn't have much space for the title, so here's the real sentence.
The London subway system, which opened in 1863, is the largest, at 394 kilometers, followed by those in Paris and Tokyo, at 199 and 155 kilometers respectively.
As you can see, there are too many commas in this sentence. That's why I asked native speakers yesterday if I could remove any of them, and some told me that the commas before "at" are not necessary and I should add a comma before "respectively". If we removed them, the sentence would have a better flow:
The London subway system, which opened in 1863, is the largest at 394 kilometers, followed by those in Paris and Tokyo at 199 and 155 kilometers, respectively.
So far, I've always thought that we need those commas because the data given is only extra information. Without them, the sentence is still complete.
So, when can we remove this kind of comma?