here in colorado, for narrow gauge, weathering -extreme with lots of UV turns most wood
very grey with streaks of remaining red or brown which fade completely eventually
often it looks like the cover of the kit mr bunce posted-the red orange streaking
i dont know if this severe geying is what happens in more humid climates with less UV-it seems to also happen near the ocean however
to answer to OP, id go mostly very light grey with a very few streaks of tan (or simply scratch/chip the wood in one or two psot, , maybe off center or on an edge, just for interest and 'wear' to reveal the raw wood) and then a light wash of weathered black (sort of a very very dark grey with a hint of brown-almost imperceptable)
for other parts of the world , less dry and less high,
i imagine darker brown or deep grey , but not black-unless it creosoted or otherwise treated
get pictures and see the color you like-this is the best way to get a good idea
i have found that layering, lightly, different colors, is really effective-each layer should be subtle and light-they build and still reveal the real wood underneath
if i were weathering an old flat car, from Colorado NG
id use a wash of medium light to light grey-id make a point to leave several planks raw-ie fresh and new and uncolored-these can have the final light wash of black however
then id use a slightly more concentrated wash , using a shader, to streak in , very sparingly, a few streaks of color-each plank individually
then the old india ink wash to bring out the crevices and grain-this will look wrong on the raw wood-imho-so onthose id use a very very thin wash of a weathered black-again straight black i think sometimes is too stark-a method for a light wash is to first wash the wood with thinner, so its 'wet' , this allow the thin wash not to be absorbed too rapidly, which can show brush streaks-the thinner wash helps to soften this
another approach, is to simply avoid uniformity among the planking -ie each plank gets a little different color, with special attention to staying exactly on the plank as the eye pick up deviations in lines very easily
fwiw, i got a toy train blue LGB flatcar a few days back, which had a blue deck
i hand painted it with Tamaya wood brown (red brown) and then streaked it with a wash of weathered black, varying among planks-its passable, at a distance even good-the red brown looks like new wood-but more what i associate with germany for some reason-the wash tones down the sheen and makes it appear a bit dusty dirty but not much
-took about fifteen minutes start to finish-the color i chose was specifically to match the wooden areas on my LGB rail trucks so that they could pull the flat and have harmonious coordinated look (so that neither highlights the differences or details of the other
hope this is of help to you
chalks are also wonderful-they are harder to get penetration into the new wood however, youmight need to use a medium such as spirits, but this also tend to make them very uniform when qapplied wet, almost like paint -it clears up a bit when it dries out and again becomes chalky