U.S. rail tracks are typically too old to handle the speed of new train technology. The limits of the rails can reduce the effectiveness of the train speeds, sometimes by more than 100 mph. The issues with the rail run from overly sharp curves to old bridges.
Why are trains so slow?
Because the majority of the track on which these high-speed trains would run is very old and does not support the train to run at a speed of more than 80–90 KMPH.