Congestion. At least in teh urban areas, having a public transport system is really really fast, will get cars off the road. It will also generate density around each of the stations. Greater density is actually excellent for the free market, as people congregate around these areas. Great choice, greater everything. The problem is that most people believe in the car economy. The effects of the 'pioneer spirit' of individual ruggedness is what propagates the car economy or even guns. They are pillars of most rural and even urban thought. So, of course things like high speed transport is relatively foreign and hard for people to get their minds around. When people think of public transport in the U.S., it's usually a place where the mentally ill, the alcoholics, junkies congregate, especially during the nocturne hours. So, there is also the fact at least in urban area that it is not a respectable way to travel. In Portland, the Max is quite popular and is continually to gain ridership. But it really needs to be a lot faster in order to support the growth of people in this area. Portland and the surrounding areas are getting mroe crowded and the roads are not able to cope. Even if you expand the roads, it just puts more people on the roads and you just end up having even more people on the roads, leading to more congestion.
A good rail network will give you more bang for buck on the same set of land. It might be interesting to reclaim some of the roads as high speed rail and see if that gets us anywhere. Sadly, Europe and Japan are ahead because they had a devestating world war that destroyed most of the infrastructure and they were able to get the kind of infrastructure that is scalable. The U.S. did not have that, and it would be very difficult to do what I envision in the first paragraph.