?: Apart from the eastern seaboard, the geography and demography of the USA (and Canada) is not suited to high speed trains. The large expanses between the widely-spaced cities are virtually un-populated by European and Oriental standards, which means that air travel is always going to be quicker and cheaper (at least until the oil starts to dry up!).
True high speed railways are extremely expensive to build, and require high passenger usage to be financially viable. They need to be built from scratch to eliminate steep grades and curves, which means building many cuttings, embankments, viaducts and tunnels. The track has to be electrified and have sophisticated signalling systems. Furthermore – and this is a novelty for North America – the tracks have to be fully fenced, with no grade crossings. In addition, the routes cannot be used for freight traffic, which is too slow – no-one has yet tried running freight trains at speeds of 180mph!
The country is way too large to have fast trains nationwide. But It does. They run 150 mph. Not the fastest in the world, but the system being built in California will be.
The US is going with more the UK model of cranking up the speed on existing lines. For instance Michigan just purchased the Detroit-Chicago corridor, part was already 110 mph and now the rest will be.
There are many cities in the US without any train service. Thus, HSR can be hard to sell. Why, for example, should people in Nashville- no service at all- get excited about trimming 30 minutes from the Boston-NYC running time?
Because they aren't practical given the heavy urbanization in the US and the distances between population centers.
The United States does not maintain the infrastructure it has because the taxation system is feudal rather than progressive. With no money to maintain what it has, it cannot afford to build what it has not.