Although grass tufts have been around for quite a while now, not many modellers seem to be aware of them and the ways in which they can be used. Because they come in many colours and there are two principal heights (6 mm. & 12 mm),  they are able to be used for most scales/track gauges.

Possibly one of the best ways to use these, is to first visit a local railway line, or look at photos of real railway lines. Observe where there is plant growth. Real trackage usually has some sort of growth nearby or even between the tracks themselves. I have seen grass/plant growth within station precincts, along the edge of a track, but under the overhang of a platform, for instance; dried grass and even green grass along little-used sidings; small, bushy growths at the foot of rail-yard buildings; at the base of signals, catenary masts and around control gear boxes, to name just a few places. Consider also, placing grass tufts near coal staithes, near the supports/walls of water towers, around hydrants and around the edge of turntables etc. As you see, quite a few places when grass tufts could be used and we haven't even left the station.