Tim

check out the Pacific Northwest Railroad Archive.  They have a number of PCRR photos here http://www.pnrarchive.org/_layouts/15/start.aspx#/Lists/Pacific_Coast_Railroad_Photos/StdView.aspx

There are a few at various historical blogs on the Black Diamond area coal mines in Washington also, like this one 

https://www.historylink.org/file/20619 

Also check the University of Washington and Tacoma Public Library sites.  Earlier gons were wood, later steel and at the end in the late 1960’s some ore cars were used.
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Cheers

Jeff Helm
The Olympic Peninsula Branch
https://olympicpeninsulabranch.blogspot.com/