After high levels of rainfall, tracks can be flooded, potentially preventing trains from passing. Provision is made in the Rule Book that if the water depth is above sleeper level but below the bottom of the rail head, trains can proceed at line speed. If the water depth is between the bottom and top of the rail head, speed is limited to 5mph, and if it is above the rail head, permission to proceed must be obtained from Operational Control. However, different companies have different policies, with operational implications for some whose policy is to reduce speed when the water is below the bottom of the rail head.