To get that effect, as Zane said, it's down to the train and not the route at the moment. If a train has ran drops in it, and the .beacon 21;0;0;x (x being the intensity of rain 70 being usual) command is used, ran will appear on the windscreen.
However, should a train have snow flakes instead of raindrops, then snow would appear.
It is therefore up to train authors for snow, and not the route authors.
But then it could be possible to substitute the raindrops in the d3d folder with snowflakes or put a mixture of both or even other things (falling trees for example!). Notice also the Birmingham X-City which, if I remember well, doesn't add things to the cl 323 folder even though it includes a few routes with snow.