It cuts the list shorter, but still not quite second place: https://en.wikipedia.org/wiki/List_of_United_States_cities_b...

Can you say that with a straight face after reading that list? Let's just look at the populations, no names:

     11,176
     66,455
     49,708
     50,005
      4,724
  8,175,133
     23,594
      2,406
     27,395
     69,781
      6,707
      7,137
     23,805
     20,832
     58,114
     75,754
     34,399
     53,926
        362
    146,199
    805,235

Now guess which ones are central cities in their respective MSAs. Excluding those, guess which ones aren't part of the NYC or LA areas. Let's do ourselves a favor and exclude Poplar Hills, Kentucky (362). There's one city left, it's part of the Boston area (which is the next densest large city after SF), and you guessed it, Somerville has a subway with a separate light rail extension under construction. The only outlier is Sunny Isles Beach, FL (20,832), in the Miami area. And Miami? It's just after Philly, all known dense cities.

So yea, it's NYC, then SF, minus a couple pockets of LA. And these are all dense places that need subways. It takes a special sort of something to claim, as the top comment did, that SF is not dense enough to warrant a subway, let alone massive large-scale investment in all modes of mass transit as NYC has.