Fine-grained aggregates must be cleaned out of railway ballast because a fouled ballast bed is more susceptible to frost heave than a clean and consistent one. The bearing capacity of the ballast bed also decreases as the ballast becomes more and more fine grained.
The Plasser & Theurer RM 80 and RM 80-U ballast cleaning machines remove the ballast from under the track for a depth of slightly over one metre. A screen system cleans the ballast of impurities and returns the aggregate of correct grain size back to the ballast bed. The RM 80-U can also screen the ballast bed under a turnout. If necessary, ballast that is too finely grained can be removed altogether from the track and dumped straight into spoil wagons (MFS-40/MFS-100).
Frost insulation boards can also be installed under the ballast bed as required during the cleaning operation.