At high speed, the draught set up underneath, and at the rear, of high speed trains tends to lift the ballast from the track. In winter, hard-packed ice and snow can accumulate under a train, then fall onto the track, rebound, and cause damage to the underside. It also sends ballast flying. If this happens when two trains are passing, it has been known to smash carriage windows. On the continent, high speed trains run at reduced speed after a snowfall. But the problem is well-known and a lot of research continues on the aeodyamics of the underside and rear of trains.