Polex,
For the snow effect, I usually wait until it is about 25 degrees F, and the snow is falling. Put the trains outside to cool down (the metal wheels will actually melt the snow, and then it freezes, which insulates the rail and makes it hard for the electricty to get to the motor).
these are from last winter, my first time using the Aristo Mikado to plow snow. The plow itself is an off the shelf Aristocraft item. It has ball bearing wheels which allows the user to put a real load in the gon. This is required to keep the snow from building up underneath.
I use a full size brick. Seems to work well. Others might use ballast, but you need something:
I am guessing you mean for like Christmas displays, right? I can't help you much with that. However, I don't think chalks alone will be convincing enough. Snow takes up space, and you might try and account for that, as well.
Maybe some of that fake, spray on snow will work.