The "snow line" option only affects shaders. Not all terraintypes have shaders assigned in 2.4.0 (IIRC; in 2.6.0 it's better). The snow you see in your first picture is part of the texture (FlightGear switches to "winter" textures when you set the season). You might even get a better result setting the season to summer and then adjusting the snow line...
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