Just like Vibrations!! Well even m not so sure but yes Air Suspension is better for higher speeds so that they balance the body weight on the bogie which helps to remain stable.
ICF Spring Dampers are used past many years n are always been a companion but tech specs, even they do have certain amount of tolerance. at a certain speed, the entire coach starts to vibrate where the weight comes on the bogie down. This sometimes results heavy shakings n vibrations n can b dangerous.