I just happen to come across this thread today and found it to be an interesting discussion. I'm a Steampunk fan myself, but the one issue I have with it is that all too often the focus is how many gears, bells, and whistles can be put on something than actually putting thought into a Steampunk Machines' design. I haven't mad many drawings for such machines, but I've made a few and offer them as inspiration for Trainz content. Under the stipulation that credit is given.
For a steampunk design contest I came up with a steam powered "Semi-Truck", minus the trailer. Here's the drawing I submitted and won first place. (The prize was more or less bragging rights, but it was all in good fun anyway!)
In retrospect there are some flaws in the design, namely there is NO brake system. I realized this after the fact, but a typical locomotive air brake system could be used with a simple 6 or 9 inch diameter Westinghouse Air Pump/Compressor like on early steam locomotives and narrow gauge engines. Or an even simpler system like Vacuum brakes, which may be better suited for such a machine that would only haul one or two trailers/wagons at a time.
Here is a more recent locomotive I designed for a book I'm considering writing. (with the theme of being a Steampunk Spaghetti Western) It's a 4-6-2 Pacific type and although is based on prototypical design, it's still an entirely freelanced locomotive. It's also intended to operate on 5ft 3in "Irish" gauge rather than standard, but it could be a standard gauge locomotive. Albeit a LARGE SG locomotive.
I've been meaning to make a cab profile, a drawing of the cab interior setup. To help along with the steampunk "feel" I wanted to have the engine have dual fire doors on the backhead, not to mention LOTS of brass trim, fittings, and copper piping. And probably an overhead throttle assembly like what is often used on Bershire engines and similar locomotives. I'm planning a whole slew of similar locomotives, a 4-8-2 of similar appearance is next on my list as well as a 2-8-4. As mentioned above I look more at practicality that bells and whistles.
However since the topic has mentioned this "Demon Train" several times, here is a pitch that I think would make a fun and unique Trainz model. Anyone here familiar with the game "Darkwatch"? It was a total flop, but the concept and theme behind it was great! Vampires and Zombies in the Old West!! Thay had their own private train, heavily armored and rather intimidating. But a Trainz model of said locomotive would be nice, and in terms of "Video Game Trains" this one is rather tastefully and believably done!
It's a bit odd as it is a 6-4-0, but it does make some practical sense with all the armor and that bladed pilot jutting out of the front. The brightness headlight also is plausible. The PRR experimented with "Lime Lights" on their locomotives. I don't know many details, but the goal was to give the engineer better visibility during night time running. And the Lime Lights worked... all too well. In fact the reason they stopped using them was that the crews of trains approaching form the opposite direction were often blinded by the brightness of the Lime Light headlamps. Just something to think about, food for thought with steampunk tech.
Anyway, the game, despite being a total failure, is still a great concept and I believe should be revamped. (No Pun Intended...) But it had inspired me to do some fan art and here is another locomotive with the "Darkwatch" styling applied to it. It's also a Cab-Forward design which is steampunk enough all its own.
The overall design is inspired by the North Pacific Coast RR's #21, a 3ft narrow gauge 4-4-0 with an experimental boiler which necessitated the cab forward design. A Photo of the actual engine is below. And again, is steampunk enough all its own.
So gentlemen, hopefully this may inspire you. I to would like to see more Steampunk content for Trainz. It just makes the game all the more fun!