Dwarven steam tanks are cumbersome, heavily armored vehicles, that are mobile siege weapons piloted by brave and sometimes reckless dwarves. These cumbersome, heavily armored vehicles can transport troops across hostile terrain and serve as mobile bunkers for beleaguered forces. Their heavy weaponry is too cumbersome to target enemy units and can only be used on buildings. Steam tanks use a phlogiston-powered boiler to fling its projectiles Steam tank crews are given tremendous respect for their skill in guiding their magnificent weapons of war.
In Warcraft III: The Frozen Throne, the older steam tank design was upgraded into the dwarven siege engine (adding upgraded weaponry, and rockets that could attack enemy air units). However, steam tanks of various types are still in use.
In World of Warcraft
In World of Warcraft, a few steam tanks are still seen, all of them ruined or damaged, but intact on the dwarf/gnome character screen. These vehicles are never seen firing or in motion, but may in the future be usable by players.
In the WoW Beta, there were steam tanks (with a rounded front, unlike the below model) where the modern siege engines are sitting, hinting that they were replaced in-game.
A version of the steam tank had a wolf head on the front like the modern 'elite' siege engines do.
The siege vehicle may also be referred to as a steam tank (or steam engine).
A siege engine may actually just be a type of steam tank, rather than a separate vehicle type altogether.