""- You know... I never wanted to be a Witch Hunter...
- Is that right, Sir? Had you any particular carer in mind?
- Yes! A steam tank driver! Think you it! CRASHING ALL IN MY PATH!
- Ach... sounds lovely... Ai! Maybe there is a wheelbarrow we could borrow!
- No, it would not be the same, because it won't have a whistle!
- Whoooo, Hooooo! Chu chu chu chu chu... you know... Whooooo, Hooooo! Chu chu chu chu...
- What are you doing Saltzpyre?
- What do you mean? I AM A STEAM TANK, chu chu chu, Whoooo, Hoooooo!""
- – A DRUNK Saltzpyre explaining why we all love Steam Tanks.
Despite being for the most part at a 1600's level of technological development, the Empire of Warhammer Fantasy Battle has METAL BAWKSES as well. They were actually invented by Dwarfs with collaboration from the founder of the College of Engineers himself, the famed Tilean inventor and Engineer Leonardo da Miragliano (a reference to real life genius Leonardo da Vinci (blessed be his name)), and thanks to the alliances between German and Scotsman in Warhammer Fantasy, Humans ended up with the plans. Dwarfs refuse to use anything that hasn't been tested in the shop for at least a few hundred years before the first field test (although newer generations of Dwarfs are rapidly using newer and newer creations, which pisses off the older Dwarfs to no end. The fact there's fewer old Dwarfs to beat them senseless for showing "Elvish Sense" may have something to do with it...) so these tend to only show up as scenario options for them or as proxies. The general design may have been simplified for the Deathroller in Blood Bowl that Dwarf teams bring though.
Only twelve were built and only eight remain to this day, the secrets behind their construction lost following Leonardo's death.
As one would expect, they're usually covered in symbols of the Empire. Griffons, skulls, Germanic flags, wreaths, and crossed muskets. 40k fans are tsundere for them, partially because after decades of seeing Rhino after Rhino the design for the Steam Tank of the Empire is a glittering prize (to say nothing of the MANLY mustache and glorious METAL hat sported by the man on top who wants to hit things with his JEWELED sword). On the other hand, anything that isn't Warhammer 40k is heresy so they can't like it. Oh well!
The Empire's steam tanks has a fair deal of issues. First of all is it's wheels. It's got four large wooden wheels, which means that it has a fair deal of weight on each of them, putting them under considerable strain and leaves it liable to sink into the mud. The second issue is suspension, or more specifically it's non existence. The Rear Wheels are just bolted onto the side of the vehicle directly. This means that every single rock, stick and slightly raised mound of dirt that the vehicle roles over will be felt as the vehicle is jostled around. Besides the fact that this means that Barf Bags will be a necessity for the crew, this will be hard on the mechanisms and will mean that stuff will be rattling around, which includes burning coals and gunpowder.
Armament has it's own issues. The turret mounted steam gun is a solid enough weapon all things considered. The frontal cannon however is more problematic. First of all it's going to recoil when fired. This can be addressed with rails and mechanisms, even though it would still be a hazard for the crew. A bigger issue is reloading. Unless it's a breach loader getting the gun ready to fire another volley under combat conditions in a cramped box is not going to be fun or quick.
The final points relate to the engine. First of all it's exposed, leaving it vulnerable to attack from the rear. Having a wooden boiler is going to restrict the engine power. Fueling is going to be an issue as space inside the thing is going to be rather cramped and you have to accommodate the coal and a stoker to shovel it in.
That said, these points avoid a critical issue: the Empire's Steam Tanks exist in a world where some variation on Spearman Steve is the standard soldier for most factions. Having any functional tank on a pseudo medieval battlefield is a big deal.
Age of Sigmar
Steam Tanks are still around in Age of Sigmar. Although no notable improvements have been made to the design of the tank itself, the forces of order actually understand how to build the damn thing now, with the human and duardin engineers of the Ironweld arsenal free to effectively mass producing tanks.
On The Tabletop
There's nothing else to say except get as many in your army as you can.
Originally, Warcraft III had a heavily armoured siege unit called the Steam Tank for the Human Alliance faction. In-game, the unit is known for trolling other factions hard by just spamming the damn thing. Due to its heavy armor and high siege damage, spamming these toys can make your opponent rage about humans being OP. Thankfully, it can only attack buildings. Oh and it can attack multiple air units if it has been upgraded (goodbye Gargoyal-chan!).
However, its name and design (along with the Gyrocopter) were changed by Blizzard for the expansion and World of Warcraft, quite possibly as a result of being threatened with having large stacks of papers being thrown at them by GW's lawyers. They are henceforth known as the Siege Engine and Flying Machine. However, in modern World of Warcraft, they're still commonly called by the older names, much like some other things we know of.
The US government experimented with this idea in 1918, building a functional prototype with a 370kw engine armed with a flamethrower, but ultimately turned it down. Internal combustion engines have an edge in power to weight ratio and did not need time to get fired up.
Other than that, the Steam Tank is actually a reference to one of the infamous Leonardo Da Vinci's turtle tank blueprint. After all, Steam Tank IS invented by Leonardo da Miragliano, whom the character itself is a reference to the real life Da Vinci himself.
The closest the Empire has ever come to victory over the Wood Elves.