I think trying to retrofit existing single family into urbanism is kind of a lost cause. Everything is built for it, and nothing is built for urban density and transit, at some point you might as well just stake a new city in the middle of nowhere (preferably off a major interstate and rail line that has water access) and build from scratch rather than having to bulldoze everything thats already there.
The Northeast US cities have the bones of urbanism still. Their grids were often laid before the car, so you have the narrow streets, housing without setbacks, capacity for mixed use and density to justify good transit infrastructure. The problem is outside New York the cities proper all depopulated - Baltimore and Philly peaked decades ago and have seen mostly population decline since mid last century. The problem is turning these places around requires a lot of investment to rebuild the decayed urbanism that is there and they both (the whole corridor) needs more transit that what it already has to support real urbanism growth when it starts up again.
These are places that have been systemically paracitized for a long time. Taxes are higher in Boston, Philly, NYC, Baltimore, etc than anywhere else in their respective states, and the actual cities are some of the only property tax revenue positive places anywhere - all that suburban sprawl is dependent on outside money to sustain all the roads and infrastructure in ways these cities are not. But that outside money often came from these cities, and we saw huge white flight last century as wealth fled to sprawling suburbs.
None of these places can really turn around the economic sackings they have endured on their own. Even NYC has huge budget problems supporting its metro. But good luck getting the broader fed to reinvest in cities in the 21st century... there are defense contractors pocketbooks to pad, companies to bailout, and techbros to give tax exemptions to.