Sodas with milk or cream were extremely popular in the 1890s, and into the twentieth century. When soda counters more or less vanished from the American landscape, sodas with actual fresh milk in them became an endangered species. When most twenty-first century Americans hear the term cream soda, they think of a brown, vanilla-flavored soft drink which doesn't contain the slightest trace of anything from a cow. In Canada the cream soda tastes the same and is equally devoid of dairy, but it's pink. A few specialty companies in the U.S. still make niche-market sodas that contain milk or cream. I also saw milk sodas when I lived in Japan, which always struck me as interesting since the Japanese really don't drink much milk compared to Americans.
Ready-made milk and soda might be a slightly foreign concept in the U.S., but its two components are available in any grocery store and luckily the recipe is not exactly complicated:
1. Pour glass half-full of milk.
2. Fill remainder of glass with seltzer water (i.e., soda.)
"Drink on the way should be milk and soda..."—Erskine, F.J. "Lady Cycling," 1897.
"I asked the proprietor of a little bar at the hill-top what the favorite drink of the bicycle riders was and he answered, "Soda and milk." " —Chandler, Alfred Dupont. A Bicycle Tour in England and Wales. Boston: A. William & Co., 1881. p.22.
"As for the soda and milk, I found that it had staying qualities, without the heaviness of bitter beer or ale. Soda is sold everywhere in England in small bottles; and I remember how satisfactory was the mixture of this that a young gentleman from Dorsetshire prepared, as we were about to part after a swift fourteen-mile run side by side out of Chichester."—Chandler, Alfred Dupont. A Bicycle Tour in England and Wales. Boston: A. William & Co., 1881. p. 23.