Nice photo Tim!  Thank you for sharing.  PQ Corporation still exists, and sodium silicate is a great example of a high specific gravity liquid (1.3 to 1.5 x heavier than water, depending on concentration).  Also called water glass, it has uses in many industries, including the paper industry and agriculture, and was widely shipped by rail.  As Tim notes, many acids and other chemical liquids are substantially more dense than water or petroleum.

Now, if only there were a decal set for this car!

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Cheers

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