High Speed is a concept introduced early on by Railcraft. Certain tracks allow you to travel at a greater maximum speed than normal rails. This however comes with a trade-off. It is dangerous. You can die if you dont design your lines correctly.
When first starting off you must use H.S. Booster Tracks or the H.S. Transition Tracks to accelerate your cart to High Speed. It should take several such rails to reach max speed.
If at any time your minecart leaves the high speed rails without first slowing down you will experience a deadly crash that will harm both you and the surrounding terrain. The same will happen if you hit anything (mobs, blocks, players, other minecarts, etc…) or attempt to navigate a turn. To prevent this from happening there are two types of high speed rails capable of slowing you down to a safe velocity. These are the H.S. Booster Tracks and the H.S. Transition Tracks. By strategically placing these rails you can slow your minecart to speeds that will allow you to safely return to normal rails or exit the cart. Though when travelling at full speed it can often take nine or ten of these rails to do so.
Another consideration is the fact that you can only travel at high speed if your line has no corners. While normal High Speed Rails can make corners and switches, attempting to traverse these without first reducing speed will result once again in death and destruction. So as before use Booster and Transition rails to slow you down before you reach a corner. Switches on the other hand only require you to slow if you want to take the diverging line. You must however use a Switch Motor or Switch Lever for this to work, vanilla Minecraft T-Switches will likely result in death no matter which way they are switched.