Going from the 1981 profile map of the Wausau area, the MILW used 4" pit-run gravel from the Wabasha area.  Depending on your era, there might be a bit of ballast, or the track sunk into the mud. 90 lb rail is prevalent. 

http://farm4.staticflickr.com/3270/2340115441_ddb29d5d17_b.jpg  Shot from Wausau, 1970.  Not much ballast, mostly mud. 

http://farm3.staticflickr.com/2107/2183777867_052fd0c5ec_z.jpg?zz=1  This image from 1987 shows a few spots of ballast around some switches, but not much elsewhere. 

As for color, maybe a buff, spots of gray, and mud.  Not sure what you have available in your area, but some sort of actual crushed rock product will work better than Woodland Scenics crushed walnut shells. 

Thanks Mike for the reply. First photo you cannot see, it says "not available". My era is 1950-1959. As I can see on the second photo, there is mud mostly. What technique for modeling mud on the yard should I use? Zip Texturing? But if I use that one, than I have to make some basic ground (what material), because a plaster cannot be sprinkled in thick layers, to hide ties for example?

Thanks again,

Jure Sporn, Slovenia

That is the plan that I have for my Los Angeles Junction.  Just get some garden dirt, if it is the correct color.  Run a magnet through it to remove any iron particles, and microwave it for a minute or so to kill any critters that might be in it.  You then just use the dirt like you would ballast or any other scenery material.

You might want to check out this thread...

http://model-railroad-hobbyist.com/node/7438

George