France has approximately 31,939 km, or 19,845 miles, of track. The USA has approximately 233,000 miles of track, or over twenty times the track that France has. But the USA is only about 17.7 times the volume of France.
The problem isn't that we haven't put effort into the rail system, the problem is that the continental US is so much larger than France. France is 543,965 sq kilometers; the USA is 9,629,091 square kilometers, or about 17.7 times the volume. By both rail-km and rail-volume, we actually have more track than France.
Volumetry isn't the sole thing. Its usage and quality is also important.
Without the TGV, don't think that people would be using train that much - save for students of course. And that is solely R&D - that didn't happen in the USA. High-speed train is french, german or japanese, and that is almost 50 years old.
It just isn't enough -- nine million square kilometers is a huge area to serve, and it is area that developed at a rate that was different than the rate rail expanded. In addition, France's population density is hugely higher than the USA; you have 60 million people, about 110 per sq-km, while we have 300 million, about 31 per sq km (and actually, because we have very high density coasts, that number is way too high for the US interior and way too low for the coasts.)
Yes, fine. However, you're thinking too globally. Rome wasn't build in one day. Begin with intra-state lines (like what's doing California: the SF-LA high speed line is a great idea), then build around it.
First TGV line was Paris-Lyon (450km, commercial opening 1981), other lines opened afterwards thanks to its huge success. And much later, french and german networks were linked, for instance.
France and the USA present two entirely different rail problems, and the same strategies can't be used to solve both. It's not practical to set up a rail grid that serves the USA in an equally distributed way -- it wouldn't save money, or fuel - it would lose money and waste fuel.
We would benefit a great deal by moving to dual-track on many routes (the US hiline is one good example... many trains sit and wait for hours in sidings because there is only one track in many locations) and of course, with all that area, hi-speed rail would be lovely - but again, with 17x the area to serve, the amount of funding we're talking about is simply staggering.
That is kind of unfunding here - but fact is that train is not fashionable in the USA, so not that much funding and ticket revenue: vicious circle.
People are much more willing to take the plane because it's much faster. Availability of high-speed train almost killed plane traffic here, and I'm sure it will be the same after the SF-LA line opens.