I have also seen references to ore (or concentrate) from Alaska being hauled in these, and all say that there was a very thin layer, or a small pile over each truck. The cars I've seen didn't have, or I didn't notice, the white lines, but most of them were BN or GN cars, and all were still in BN service. Also, I wasn't looking for white lines on them, but that one is rather noticeable...
That white line is far too high for any type of ore loading, and probably for any type of rock at all. Maybe it has something to do with somebody's loading/unloading system, or it might just be to distinguish the GBRX cars from other owners at a glance.