Even ignoring the money, if you try and do a project like that you are going to get slapped in the face by all the same "cutting apart muh neighborhood" rhetoric that gets used against highways. Grade separation and "just paying those people to go away" are both expensive enough to be non-starters.
Passenger railroads are narrower than urban highways. The US-101 freeway cuts through Echo Park and Westlake in Los Angeles, taking up as much as 330 feet of width, counting on/off ramps. California HSR has trench sections specced as narrow as 72 feet[1], and most of its urban rights-of-way are under 100 feet wide.
Walking next to the 101 in Echo Park you can see how it so starkly divides what was once a single connected neighborhood. The light rail lines (about ~40-50 ft to cross) in other neighborhoods don't give that impression.
[1]: https://www.hsr.ca.gov/wp-content/uploads/docs/programs/eir_...
You're being dishonest or ignorant. The fact that you compare max-width of one to min-width of the other rules out one option. A pit is a pit. You're limited to crossing at a few specific points no matter how narrow it is and traveling to those points accounts for the bulk of the distance covered. The physical width only matters if you're evaluating the neighborhood for visual appeal and not actual livability. "Quiet most of the time" doesn't really count for much because people acclimate to the background noise levels and that one train per hour is just as jarring as that one motorcycle with the insane exhaust per hour. At least with subways and airports it's every couple minutes so you get more used to it.
All corridors are troublesome.