The use of extruded polystyrene foam boards in track structures requires that high thermal resistance is retained in the harsh loading environment of the track. To retain low thermal conductivity, the frost insulation board must be able to resist the absorption of water despite minor mechanical damage. The replacement of the blowing agent used in board production over the last 15 years, due to environmental concerns, has also decreased the thermal resistance of the board. This paper discusses the field and laboratory studies conducted to evaluate the sustained performance of XPS boards. As a result of the project, the dimensioning value for the thermal conductivity of XPS frost insulation boards was changed to 0.050 W/Km and stricter compressive strength requirements for the boards under high axle loads were introduced.