The concept of railway-mounted artillery can be dated to the 19th century, however their shining moment came during the First World War, and the most notorious of these was the German manufactured "Paris Gun" which showed up in 1918 and was able to shell that city from as far as 75 miles away.
This article appeared a few months prior to the war's outbreak and concerns the railway gun that the French had on hand at the time: 7.87 inch, 6 inch and 4.7 inch howitzers which were intended for coastal defense. By 1916 both sides in the war would be deploying enormous rail-mounted naval guns, capable of delivering a far larger blow.
Click here to read about the U.S. Navy railroad guns of W.W. I.