The lack of between the train and the tracks allow the train to reach unbelievable speed.

Friction

Explanation:

Friction is the force that is created when two objects rub against each other. All objects are rough, even though it might not appear that way, on the microscopic scale. These little grooves and ridges get stuck against each other when two objects are moved against each other, and they slow down motion. If there is no friction between a train and its tracks, then it can continue to accelerate without slowing down. Hope this helps!