The syphoning of water from the boiler into the steam pipe, caused by too high a water level or by certain chemicals used to treat hard water. If water gets into the steam pipe it will affect the performance of the superheater by reducing the ability of the steam to dry properly and, if it reaches the cylinders, it can damage them and the motion. In extreme cases, cylinder ends have been blown out, valve gear bent and locomotives derailed by the carry over of water into the cylinders.