Engines is a d20 Modern-compatible supplement detailing how to build the fantastic mechanisms of Victorian fiction. From the publisher:

The Imperial Age: Engines is the ultimate Steampunk technology sourcebook, designed to aid Game Masters who wish to add fantastic machines (known more commonly in the Imperial Age as "Engines") to their Victorian-era d20 campaigns.

This supplement gives the GM all the tools needed to design and build any item of steampunk technology, from vehicles to weapons. Airships, Aetheric Wave Cannons, Giant steam-powered Automatons...all of it can be created with these rules. All in one place, at your fingertips.