Bob the #1 ingredient would be sugar, originally granular sugar in boxcars. In later years granular sugar in covered hoppers (though it would be late in the steam era) and/or corn syrup in tank cars. In far lesser amounts you would have fruit juice or sugar substitutes (in the case of diet drinks). Soft drinks may also contain caffeine, colorings, preservatives and other ingredients. Most likely shipped in boxcars in barrels or containers, but tank cars for some ingredients that come in a liquid form. The carbonation comes from adding carbon dioxide to the water, which I believe is shipped in tank cars, or in CO2 cylinders that may be shipped in boxcars. Water of course was almost always a local commodity.
Once you have ingredients you must put them in something: ie bottles, cans, or pressure tanks for fountain drinks. Then there are containers for the bottles or cans, early were wood, later cardboard, labels, bottle caps. All of these items would come in boxcars.
