The hyperloop was created by NemesiS_ITA as a fast way to travel across the map. When the main hyperloop was created the map was much smaller. Until the expansion to 4300x4300 the hyperloop was sufficient for the whole map. After the expansion the Player made hyperloop was created to reach the new areas fast as well.
The main hyperloop is marked on the dynmap by pink lines. To use the hyperloop you will need a boat and an emerald. Make sure you have a boat before clicking the button, you will be transported right into the hyperloop and wont be able to make/get a boat.
At the main hyperloop station you can buy boats for 1 emerald, but there are also echests at all the stations. You can walk/run across the ice but it will take much longer then riding a boat.
At the outer exits of the hyperloop you can just walk out by stepping on the pressure plates at the end of the line. When you're going to the center of the map you will need to hop onto one of the pressure plates in the corners of the room to be transported outside.