Trains navigating sharp curves use automatic track lubricators that spray grease onto the rails to prevent the ear-splitting screech of metal wheels grinding against metal tracks. Without this lubrication, the friction from a heavy freight train taking a curve would be so intense that it could actually shave metal off both the wheels and rails, requiring expensive replacements every few months. These trackside lubricators are triggered by passing trains and must use biodegradable greases in many countries to avoid environmental contamination along thousands of miles of track. The grease needs to be sticky enough to cling to vertical rail surfaces in rain and wind, yet slippery enough to reduce friction by up to 90% when a million-pound train passes overhead.