My guess - based on the origin of the cars and Hercules' business in printing and paper chemicals - is that they were hauling paper-making chemicals.

Think about it: If they acquired a company that used and owned tank cars, wouldn't it be logical for them to continue those tank cars in the same service?

With respect to nitroglycerin, I can assure you from my time working with nitroglycerin at Hercules' Magna Utah explosives plant that it would be impossible to transport nitrogylcerin in a conventional tank car. I suspect you'll find that it was also illegal.

KL