The sealing characteristics of tie pads on concrete-tie rail seats directly influence the intrusion of moisture and fines, which are two of the four primary causes of concrete-tie rail seat deterioration (RSD). The amount of leakage allowed by the tie-pad seal also determines the response of rail-seat surface water to wheel loads. A laboratory test apparatus and procedure were developed and implemented to measure the surface water pressure caused by applying normal loads to different tie pads. The measured surface water pressures were also used to estimate the potential velocity of water at the pad-rail seat interface. Results from the laboratory tests suggest that an effective tie-pad seal causes the surface water to become pressurized under load, whereas an ineffective tie-pad seal allows the surface water to flow under load. Pressurization may lead to hydraulic pressure cracking of the rail seat, while high-velocity flow may lead to hydro-abrasive erosion of the rail seat. Relevant sealing characteristics of the tie pad or assembly include the bulk flexural rigidity; the hardness, elasticity, roughness, geometry, and durability of the surfaces at the tie pad-rail seat interface; the contact stress at rest (resulting from the clip toe load); and the contact stress under load (resulting from the wheel load). Further research is needed to understand which RSD mechanisms cause the most damage in the field and, therefore, should govern tie-pad design.