â€œGrassâ€� is a pretty broad term, but if youâ€™re modeling in G-scale, the short answer is Creeping Thyme. Up close it doesnâ€™t look anything like grass, but it spreads out in a dense mat, grows fairly quickly and resembles a grassy patch from a distance. You will get flowers â€" I do, at least, in the Spring, but they donâ€™t last long, and while they do last, they are quite nice to look at.
Elfin thyme â€" smaller leaves but comparatively sloooooow grower
Wooly thyme â€" interesting plant but looks more like ground hugging vines
Moss sandwort (Irish) â€" good for 7/8ths but a bit tall for g-scale, and too uniformly green
Dianthus â€" great for representing tall wild grass in 7/8ths but in G scale it could get 12 feet high
Scotch moss â€" looks too much like, well, scotch moss (and too recognizable as such)
One of the strange things about garden railroading is that there is no go-to plant when you want to represent something as ubiquitous as â€œgrassâ€�. To me, itâ€™s more useful to think in terms of, how do I represent a meadow? In my experience, this can be done by mixing a variety of the above groundcovers (and others). The clincher comes when you toss in other small perennials which do an amazing job of standing in for shrubs, bushes and thickets. Thatâ€™s when it all comes together.
I am in zone 6.