High-passenger-load high-speed rail would dramatically reduce the impact of the passenger transportation sector on energy consumption and CO2 emissions.
Matt Yglesias pointed out that the federal government already has designated "high speed rail corridors" across the nation. Building 12 of these corridors was estimated in 1997 to cost $50-75 billion over 20 years so the $8 billion in the stimulus won't get the whole job done. But it's close to the right order of magnitude, and far far better than the $1 billion/year or so that the Federal Railway Administration has been managing on lately.
I've ridden the Amtrak Acela between New York City and Washington DC a bit (most often I take the slower "Metroliner" train because it's quite a bit cheaper); even that could be a lot better and faster. There are sections of track that currently just aren't suited to high speed, and even the top speed of 150 mph is relatively slow compared to European and Japanese high speed rail. So - one hopes that the new projects on the new high speed corridors will be "done right" - at least after all reasonable alternatives have as usual been explored.
So, kudos to whoever got this into the stimulus package, and I'm looking forward to more rail travel in my future here in the US!