The only problem with using brush hairs is that you get that look of them growing in clumps like someone planted them. Grass of course grows all over in a more even pattern. So, I like to get very cheap fake fur (the attic, fabric store, garage sales, wherever) in a brown color. It is usually lying in a certain direction, so I comb it in the opposite direction to get it to stand up, and then spray paint it an appropriate color. The brown will generally show down deep, which is good, and then later I go over the tips with some sort of highlight color, say yellow.
When using vehicles in this setting, making track marks are very easy. Just take a soldering iron to where you want the track marks and the fake fur melts into very nice clumps like the ground was turned up. Whatever you don't like can be pulled away.
Hudson and Allen sells something like this too on a matt. There is also a European RR modeling product (Heiki grass?) that comes on a matt too that many people like. I have not seen it though. I think it is used by 1/35 modelers for more of the shorter, wild grasses, as most of what I seen is that the grass is lying down, and not up like you describe.