Machinations is a simple steampunk RPG. From the author’s description:
Welcome to the Eternal City, the home of the Machinations RPG. Take on the role of an Adventurer, an Alchemist, an Artificer or even a deathless Automaton. Band together with others to form a Free Company and then hire yourselves out to perform acts of sabotage, espionage, assassination, extortion, blackmail or plain old robbery. All in a world a short step from our own, where reason has been lost, empires exist to be built and strange technology dominates all walks of life. It is a world of sinister intrigue, awful power, terrible events and dark portents. There is also beer, song and glory
The Toposolitario site has a nice selection of paper models of mecha suitable for steampunk, pulp or wierd WWII games.
You can download a pdf of the Albatross steam flyer shown above.
One of those great ideas that never was is the mole machine — a transport with a giant drill on the front that carries troops under enemy lines.
Here are some instructions for building one for your 28mm troops.