Higher freight traffic in the U.S. has partially something to do with it. Buffering strengths here require that a passenger train be able to withstand 800,000 pounds of force without deformation, leading to trains that are nearly twice as heavy (and thus slower) than trains in other parts of the world. Europe, on the other hand, doesn't have quite as stringent requirements, and instead of requiring rigid frames they mandate crumple zones, which are arguably just as safe. MetroLink in SoCal has started employing something similar on their cars.
To give an idea just how big freight traffic is in the U.S., freight by rail is something like 1.7 trillion ton-mile (39.9% of freight by ton-mile). The total across all modes in the EU is 1.4 trillion ton-mile, of which rail makes up 17%, so only about an eighth of the U.S. in freight. This has led to fairly different rail systems.
To give an idea just how big freight traffic is in the U.S., freight by rail is something like 1.7 trillion ton-mile (39.9% of freight by ton-mile). The total across all modes in the EU is 1.4 trillion ton-mile, of which rail makes up 17%, so only about an eighth of the U.S. in freight. This has led to fairly different rail systems.