High Speed Rail allows Minecarts to travel 2.5 times faster than normal, while moving in a straight line. Additionally, attempting to leave these rails or make a turn without first slowing down will likely result in an explosion which may kill you.
Collisions are bad too as even a single dropped item on the track is enough to cause an explosion.
The speed of the High-Speed Tracks is even greater than that of running with Quantum Leggings boost.
You can use High Speed Transition Track to slow the speed by going over them in direction opposite to arrows on them when they are powered by redstone. Also H.S. Booster Track can be used to slow you down if not powered by a redstone signal. About eight of either of these slow the cart down to a normal speed, so you should then be able to execute a turn without the track coming apart.