In rainy season the dust in the ballast becomes mud and comes up by suction from below the rail joint. Such a joint is called

A pumping joint occurs when fine ballast dust mixes with water during rainy seasons to form mud, which is then sucked up through the joint gaps under the repetitive loading of passing trains. This pumping action creates a visible mud extrusion at rail joints. The term 'pumping' refers to this suction and extrusion phenomenon, not to the presence of water alone.