For those who use matmatic, here's the shader:

def inrange(x, lo, hi, k): return Clamp((x - lo) / k) * Clamp((hi - x - k) / k)

w = .5

x = 2 * Abs(U - .5)

y = Abs(V - .5)

xw = x - w

r = Blend(y, Hypot(xw, y), 0 <= xw)

uo = U - r

mt = .1

soft=.25

track = rgb(90, 40, 15)

track = Tile(track, 1, track, 1, 1, .02, 1, mt, 0, soft, U_Offset = uo)

intrack = inrange(r, .257, .425, .001)

grass = rgb(40, 60, 20)

color = Blend(grass, track, intrack)

s = Surface(color, .8, 1, 0, .01)

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)