Ballast cleaner

A ballast cleaner (also known as an undercutter) is a machine that specialises in cleaning the railway track ballast (gravel, blue stone or other aggregate) of impurities.

Over time, ballast becomes worn, and loses its angularity, becoming rounded. This hinders the tessellation of pieces of ballast with one another, and thus reduces its effectiveness. Fine pieces of granite, like sand, are also created by attrition, known simply as "fines". Combined with water in the ballast, these fines stick together, making the ballast like a lump of concrete. This hinders both track drainage and the flexibility of the ballast to constrain the track as it moves under traffic.