Potash is for export mostly to Japan via a terminal in the Portland, OR area.
As for the PS, been wondering that myself. There are a couple of other similar "codes" that
you hear from time to time.
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There have been a couple of unit trains of Potash from another source spotted recently using a mix of dark brown covered hoppers with standard AEX covered hoppers mixed in. Small stickers with a company logo of some sort are on the dark brown cars, but only the word potash is big enough to read from trackside.
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Lately UP has been running quite a few grain trains south out of Eastport. However, today
the UP was pretty dead, I spent most of the day today in Cheney and nothing was run, DS 41 told a track inspector that there was only one train called out of Hinkle so far. BNSF in Cheney was quite busy.