The coefficient of adhesion is highest when the rails are dry.
The coefficient of adhesion is influenced by various factors, including the speed of the train and the condition of the rails.
As the speed of the train increases, the coefficient of adhesion typically decreases due to factors such as centrifugal force and the reduced amount of time the wheels have to maintain contact with the rails.
This is because the higher the speed, the greater the forces acting on the wheels, which can lead to slipping and reduced adhesion.
The condition of the rails can also affect the coefficient of adhesion. When the rails are wet and greasy, the presence of water and other contaminants can create a barrier between the wheel and the rail, reducing the frictional force and the coefficient of adhesion.
Similarly, when the rails are covered with sand or other debris, it can create a rougher surface that increases the frictional force and the coefficient of adhesion.