One problem is population density. The US is geographically twice the size of the EU, but has three fifths the population. Germany is the size of Montana, but has 90 million people. Typical travel distances are short, and the number of travelers on any given day is high. Fixed costs would be proportionally higher for a US service as a result.
Local transport upon arrival is a big issue. Most European cities have public transportation systems that are effective enough for large portions of the population to not own cars. This is only true in a very small number of US cities (probably fewer than 5). For many people, any cost savings from taking a train instead of driving would be offset by the cost and inconvenience of securing transportation at the destination.
A final issue is that very long distance train travel still takes a long time compared to air travel. Using a generous estimate of a 100mph average (remember, the trains have to make stops), a high-speed train would take 20 hours to get from LA to Chicago. I think few people would opt for that over a 3 hour flight even given a significant difference in price.