You're comparing a vast corridor connecting half of the Atlantic Seaboard [0] with individual cities.

The US East coast between DC and Boston has 3x the population density of France (and has a larger total population). Yet France has the TGV, with many long stretches of 300 km/h traffic, and all the Northeast US has is Acela.

0. https://commons.m.wikimedia.org/wiki/File:Boswash.png#mw-jum...

A big difference you're missing is the I-95 corridor is linear, so you have huge pockets of density all close together along just one single highway / train line, which makes it easier to connect than randomly spread out. Philly is kind of a podunk city for the northeast yet it is denser than metro London, which is arguably one of the top 3 cities in the world. You are kind of proving my point!