Population density isn't really an issue. At high densities subways and trolleys work well, most major cities in the 40s had one or both. As density goes down light rail gets into the picture. Light rail works well at typical suburban densities. As you move to low densities passenger rail and high-speed rail works well and is easier to implement because the long straight routes that favor such traffic are easier to establish.

The organization would likely be a hub and spur system with central hubs being linked by high-speed rail with lighter rail systems linking smaller towns and on down to neighborhoods. Ideally the entire structure would be mirrored by a freight/mail/cargo system, likely based on standard shipping containers.

Trucks, buses, cabs and rental vehicles would fill in the gaps.

One of the biggest issues is psychological. We Americans like to think of ourselves as different and special. Europe isn't really all that different than the US. They have sparsely populated areas and the distances, say from Poland to Spain, or northern Sweden to southern Italy, functionally as open as cross-country travel here, is in the same ballpark in mileage as a trip across the US.

Also it has to be noted that a major shift to nuclear may end up just kicking the can down the road instead of solving the problems. We are talking about Peak-Oil now but we might be talking about Peak-Uranium in thirty years if we lean on it too hard. The lighter we step the longer the resource lasts.

Of course we could extend nuclear semi-indefinitely by using breeder reactors. The problem being that breeder reactors produce weapons-grade materials and the more practical designs, in terms of productive breeding ratios(above 1:1), are more exotic and potentially risky. Reactors using liquid sodium as coolant has obvious issues. Thorium breeders offer theoretical advantages but these have never been built as anything but tiny experimental units.

The obvious answer is fusion power. And as soon as a commercially viable fusion reactor opens I will put my jet-pack down and hop over to the opening on my flying car. I'm not anti-technology. But fusion energy has been a distant promise for sixty years now. It will be great if/when it arrives so we should fully fund research. But you don't base a national energy policy on a technology that hasn't been established, fusion or thorium.

As far as nuclear power goes England, France and Germany are well ahead of the US. We still get the largest single chunk of of our electricity from coal. Of course Europe leads in Nuclear, solar, wind, rail, high-speed rail, and both the availability and price of broadband communications. Not too shabby for a bunch of knuckle-dragging socialists who live in freedom-hating hell-holes of government oppression.