Some suggestions for weeds–I haven’t specifically tried this between the rails, but it makes much better weeds elsewhere, and I’m pretty sure it will work for weed grown track as well.
Go get yourself a piece of cat tail!
Hmmm, before you get scratched too badly, maybe I should be more specific–what you need is a Cattail, the common wetland plant that puts up a puffy, brown, tubular seed stalk at the end of each summer. A half dozen of them will do the weed sections for a good sized layout.
Peel the cattail apart, and it unwinds in your hand. At this point, you have a whole bunch of parallel fibers, coarser than fake fur and much more weed like in appearance. You can plant them in a puddle of white glue (better yet, my favorite is liquid latex) and LEAVE THEM ALONE until the glue or the latex dries. That will be at least overnight.
Once dry, pull out or brush out any fibers that won’t stay put. Pretty much the same deal you work when planting any fibrous scenery material. But here’s a difference–you can singe the heads of the grass you just planted with a cigarette lighter to create a variegated, late-summer dead grass effect. A lot of the weeds you see along RR tracks are only green for a month or so in May-June, and then appear in various tones of brown the rest of the year.
Best technique is to rub a damp paper towel over the surface before hand, to keep the whole clump from burning. And obviously, you remove any other fuels such as solvent cement, etc. before lighting the ciggy lighter.
Try this on a small scale, on a test piece of wood, and I think you’ll be impressed. Especially if you dye the latex with a bit of earth-toned acrylic paint first.