I've cut up tiny bits of brown paper bag, envelope paper, etc. as litter, and small strips of black tape to represent steel strapping. This is all scattered along the track and fence line, more heavily so in front of the loading doors where debris and stuff would be likely to be swept out of boxcars. There's a broken wooden pallet or two laying along there as well.
I've moved the warehouse over to the workbench to get a close-up of a couple of Lance's doors photos that I used. The little bit of grime streaking down the walls is done with thinned artist oil paints and a flat brush.
With half of the foam board roof removed for clarity, you can see the plywood base and scraps of 1 x 2 that I cut for roof and wall supports. This back side of the structure actually is situated along the layout aisle.
I wanted reasonably modern truck docks, so I searched google for photos and found one I liked. I printed it 3 times, cut them out and applied them to the end of the warehouse. The trailer parking area is spackling painted with weathered black acrylic.
One more shot from across the street, with a high cube boxcar spotted at door #6.