For the wagon tracks I know how I would do it in PS maybe someone who knows gimp better could chime in and say if it translates or not?

1. I first made a custom brush that is just to dots set apart and then painted on a new layer with my brush control set to directional. This is probably just as easily done by copying the line and placing it again or just drawing two lines, but this way if I wanted more complex wagon tracks it would also work
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and here is what I mean about a more complex shape (I didn't have to calculate the distance between the lines at the corners that was all done by a single brush).
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2. I lowered the fill to 0% (fill is the visibility of what you've drawn but not the effects you apply to it. And added a Bevel Set to down (to make it look like the section is sunk rather than up to make it look raised). I changed the blend mode for the highlights to overlay rather than screen, and lowered the opacity of the shadows a bit down to about 50%.

3. I then fiddled with the setting a bit, including doing a custom shape for the bevel but these fiddly bits are things best messed with on your own. Play with bevel shape, depth, opacity of the layer etc. till you like the look.

Here are my results:
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Sounds like a lot of steps, but once you get the hang of it it all takes only a couple of minutes at most. Learning beveling tricks will be of infinite help to you in the future for battlemaps.