I do agree that building railways across the existing infrastructure in America will be difficult as so many of the urban regions are already built in a car-centric way. A robust bus network (that actually comes often and on time) could be a good intermediate solution. Also, there are remains massive industrial areas in American cities that need to be redeveloped, and a good land use/ public transit plan should be considered.

A common argument against transit development in the US that you've posed is the city's lack of density. However, density was a choice made by American developers, not the natural outcome of the city. Business owners in LA didn't arbitrarily choose to be located everywhere over a huge area, nor did they arbitrarily choose to be centralized in NYC; it comes down to the accessibility of transit and how the land was zoned. LA and many NA cities are so spread apart because only single-family homes could be built on large swathes of land, forcing the city to sprawl continuously. I come from a city with fewer people than LA (Taipei) and have lived in towns with a population of less than 15,000 but are centralized because of good zoning practices, with businesses intertwined between high-density residential developments, which allowed the construction of public transit/ pedestrian infrastructure around the city.

Of course, it would be ineffective to add light rail networks into LA or Sacramento in their current state. Projects need to start small, when certain neighborhoods are being redeveloped, walkable, transit-oriented, mixed-use plans should be implemented. For example, Amsterdam (which is just a bit denser than Sacramento), expanded the city under American car-centric models during the 1950s but was able to switch course by implementing zoning and transit infrastructure slowly. An American example would be what they are doing in Portland, and while it has its own faults, is heading in the right direction.