You seem utterly impervious to the fact that an underground subway line is so prohibitively expensive it will never happen in a million years. There's no surface right of ways to use like they did with the HBLR, and an elevated line is also stupid expensive. Your desire for it doesn't make it any more practical than the often raised idiotic idea for a Hudson pedestrian bridge or tunnel.

There's zero reason to think building subway would be cheaper in Hudson county than in NY.

The Most Expensive Mile of Subway Track on Earth

https://www.nytimes.com/2017/12/28/nyregion/new-york-subway-construction-costs.html