What could that possibly be? I don't know of anything even on the horizon. Rails and steel wheels are more energy efficient but far more costly to build and maintain. Other nations use rail well by ... not having sprawl. Suburban sprawl essentially rests on the private automobile. As the gp essentially said, the draw of sprawl is a big house and a car, which can be nice 'till your whole planet is on fire and you're choking in rubble (I live in California, can you tell - But Texas will notce climate change too, if they haven't). A road is a just a flat piece of ground, it's pretty hard to find something simpler. Freeways are cheaper to build than railways. But the cost of rebuilding gets huge.
So basically, there are not solutions to these problems. We've basically hit Decline of the Roman Empire levels here. That empire took hundred of years to fall. I'm hoping things last past my lifetime.
The problem is this is a fundamentally unsustainable proposition. The US is hitting those problems now and it's not pretty. It's not just gridlock but problem that once things stabilize, maintaining sprawl costs more than maintaining a compact city[1]. The roads and the schools in the suburbs are often good only when they're new - once they age, the problem become visible (California is harbinger for the rest of sprawl land, this state has the older sprawl, remember). Poor services can cause an area of sprawl to become undesirable and then it's hard to recover - public transit is hard to add when distances are large but you can still wind-up poor people lacking vehicles in the hinterlands.
[1] https://usa.streetsblog.org/2015/03/05/sprawl-costs-the-publ...