Basicly, computers can simulate every aspect and enviromental condition the coaster will have or face. When it is all put in the computer, the computer will tell you how many g forces or the max speed the train will go at full capacity or zero capacity, they can even take into consideration the weather and other aspects like how the train would run at the end of the day compared to in the beginning of the day. The only way it would come out differently is if the data was entered incorrectly, or the coaster was not built the way it was done in the computer. The max speed could go over 55MPH but it would be slowed to what ever speed it needs to be when it goes over the brakes and it would only go about 1 or 2 MPH faster.