1. much of that high US traffic volume comes from long-range transport bewtween distant areas of high population density (especially transcontinental), while EU has a more throughout high population density;
   
2. the ideal distance for rail freight transport would typically cross borders, and still a lot of technical and bureaucratic differences constrain fluid cross-border traffic (but this is changing);
   
3. US railfreight has track priority above US rail passenger transport, in every sense (including savings on track maintenance that would be intolerable to maintain our fast and smooth-riding passenger trains).

Re whether Europe or the U.S. is more vulnerable to a liquid energy supply shortage, one thing to keep in mind is the "hardening of demand" effect that goes along with conservation. Since we waste so much over here, we could implement a substantial degree of conservation with relatively small economic effect. But if you're already taking a bus, or carpooling in a small diesel car, or riding a bike or walking to work, how do you further reduce your personal need for fuel?