Two things that I know of that work for snow is white plaster and fine marble powder. Don't just put down hardshell scenery and leave it white, it will be too white to be believable. Scenic the layout as though it were summer, except don't put foliage on any trees that are not evergreens. Sift the "snow" on as the last layer of scenery material. the marble dust sifted over the white plaster gives the entire scene some sparkle. I bought marble dust from a craft store that was going out of buisiness, but I think it is available from craft stores. Is the yard you are modeling located in the mountains? Generally yards are located in flat areas. Major yards have as much asphalt between tracks as they do tracks, particularly in areas where railroad workers work on trains like rip tracks, service facilities, etc. On a hump yard, expect the areas on either side of the tracks all the way up the hump and at the top of the hump to be paved for a smooth surface for the brakeman to cut cars loose. There may not be asphalt between the tracks in the destination yard at the bottom of the hump because it is too dangerous for men to work down there with cars coming down the hump, but there will be paving down to any retarders and switches from somewhere in the yard to allow vehicles to get to them for necessary maintainance and repairs. Don't model a lot of ballasted track. Generally the high ballast we model is found on the mainline and branch lines. Yard tracks are generally put down directly on the dirt, or on a layer of ballast that is generally close to grade level. If you are modeling a working rail yard, don't let snow cover the whole thing. Everyday operations of the yard are going to make paths in high pedestrian traffic areas. Rails will be kept clean by trains moving over them. Snow won't fall under railcars and locomotives; and since they are kept moving by the everyday working of the railroad, random bare areas will be visible where a car or cut was parked during the snow storm and then moved after the storm passed. Switch points will be kept clear because they need to work to switch the cars around the yard. I will probably think of other things to mention later and others may chime in with some things I have missed.