Charlie,

I’ve never seen any reference to silver, grey, or a galvanized paint anytime during the 50s or before. Exceptions do occur, but the B&O seemed to cover things with car cement for water or rust proofing. Silver roof paint did appear on some MOW and company cars in the 60s, but haven’t seen it on boxcars. 
 

Bruce D. Griffin
Ashland, MD
https://bomodeling.com/blog/