My guess, and it is just an educated guess, would be painted with silver paint.
My thought is to use galvanized one would have to size and drill all the parts, then galvanize in order to provide rust protection. And lastly assemble. Seems that fabrication then paint would be a much easier ways to go. Less expensive too, and railroads don't like to spend money.
Signals on mainline railroads were made of cast iron and steel, then painted. Today after 75 years they are being replaced by new signals made of aluminum.
The tanks were probably made by Eaton Metal Products at 48th and York Street in Denver. They had contracts with most of the industries in Denver. They were 14 gauge hot rolled steel and painted in a red oxide color.
They are, to the best of my knowledge, still made.
We had a similar one at my Parents dairyfarm in the 70's, 'twas painted yellow.
Here's one a little closer to Silica that has legs of unequal Angleiron and different bracing to your examples and also has a drumrack between the legs for decanting said drums.
Just remember these thing were not set level, but supposed to be slightly sloping downwards with the outlet at the higher end to trap any water in the bottom....well at least that was here in NZ, you guys might not ever have had thought about that.