The invention of the steam locomotive and the railway, meant it was possible for the large scale movement of men and material to the battlefield in numbers which dwarfed those of previous conflicts.
One of the innovations was the use of armoured trains, in which the train became not just a transport vehicle but a combat unit in it's own right.
Armoured trains were used on many fronts from the American Civil War up to and including World War Two where they were found mostly on the Eastern Front.
Soviet armoured train;
Obviously, there is one great drawback in that they are tied to the railway tracks, and as petrol/ diesel combat vehicles are much more flexible tactically the armoured train disappeared.