Siege engines were replacements to the dwarven steam tanks of the Third War. The sturdy dwarven siege engines were a new addition to the Alliance forces during their battle with the Scourge. Though many believe the siege engine is an unwieldy vehicle slowed by its thick layer of armor. The stalwart dwarven pilots have even been known to ram enemy structures at high speeds and reduce whole fortifications to dust under the siege engines' steel treads. Some Siege Engines were upgraded with anti-air rockets to take out enemy fliers.
Siege engines were used as an upgraded replacement in Warcraft III: The Frozen Throne replacing the older dwarven steam tank model in Warcraft III: Reign of Chaos. The new vehicle included upgraded weapons and rockets that could damage enemy air units.
In World of Warcraft
In World of Warcraft, siege engines can be seen guarding Alliance lands and outposts. These vehicles were never seen firing or in motion. Steam Tonks were likely used to give players a taste of these vehicle's animations and controls. Steam Tonks are a remote controlled toy version available at the Darkmoon Faire. Players can use them to fight other steam tonks.
Some siege engines that appear to belong to higher ranked individuals, like Pilot Stonegear of the Ironforge Siege Brigade, have more exhaust pipes, different pistons on the sides, and wolf heads in the place of the standard eagle. A "drag" verson also exists which has wheels in place of the front roller and a sleeker body, though the removal of the crushing roller (which is the defining definition of a siege engine) on the "drag" type makes it a steam tank (which is odd, because in WoW there is normally a bigger visual difference between the two).
The siege vehicle may also be referred to as a steam engine (or steam tank).
Siege Engines as vehicles in WoW
Siege engines that can be operated by players appear in a number of locations:
Technically, a siege engine may be a subtype of the "steam tank" rather than a separate vehicle type altogether. This is supported in World of Warcraft by the mob versions of the siege vehicle alternating between the two in name.