High-Speed Rails allow carts to travel at a higher speed. Note that the carts need to be sped up by High-Speed Transition Track or High-Speed Booster Track, and you will need several to get to the max speed. Note also that high speed rails are dangerous! If any turns are made, the cart collides with anything, or it runs on any normal track without first slowing down to normal speeds (by either a non-powered High-Speed Booster Track or by a High-Speed Transition Track facing the wrong way), there will be an explosion and the player will die. WARNING: There is also a bug in the coding that will occasionally cause this on slopes. Even just a single item lying on a track can make an explosion happen. So be sure to go through every now and then to pick up any items that might be lying around.
The speed of the High-Speed Tracks is even more than that of flying with a ring from Equivalent Exchange or running with Quantum Armor.