Just to see how it would turn out, I decided to knock together a Z-scale three-dome tank car from the tube stock and assorted bits and pieces I ordered a while back. I've come to the conclusion that it'll be entirely too much trouble to build every single car from scratch, so I'm focusing instead on building individual cars only until I get one of each type that is good enough to reproduce. Once I've accomplished that, I'll make a mold of each prototype and cast the rest out of resin.
That approach offers several advantages over my original plan. First, it gives me a lot more freedom to experiment and refine my technique; I have enough materials on hand to build five of each type of car I had planned, and if I got my hands on a few more end caps for the tanks I could stretch that to eighteen. Second, it reduces the overall workload since after the first workable model is done the rest will only require final assembly (application of handrails, ladders, brakewheel, etc). Third, it opens up the possibility of making more cars than I need for my own purposes and selling the extras to offset the cost of the project.
Advantage #3 can safely be stowed in the 'counting chickens before they're hatched' department for now. This is, after all, my first attempt at serious modeling, and at this point I'd be foolish to hope that what I'll come up with will be up to the standards set by other Z-heads. Some of them set the bar pretty high, and all of them have a great deal more experience.
Still, I'm very optimistic based on my progress so far. Working entirely with hand tools and eyeballing most of the measurements, my biggest mistake thus far has been setting the center dome half a millimeter off-center. Unfortunately, that mans I didn't get it right on the first try. The spacing between domes has to be accurate; as it stands the gap between the first and second domes is 5mm while the gap between the second and third is 6mm. The difference works out to several scale feet, and it is noticeable when viewed from the side.
Here are a few fuzzy pictures that I took with my ipod:
I haven't figured out what to do about the dome caps yet. I may have to do for those what I intend to do for the entire run of cars- that is, experiment until I get one that looks right then cast a whole bunch to glue on. Since those parts don't interfere with the rest of the build, they can be left off until I get them figured out.
Even though this particular prototype is defective, I still intend to finish it. I already have a few ideas on how to improve the design (using L-beam stock for the chassis instead of bar stock, for example), but getting a perfect result is likely to involve a bit of luck. The holes in which the domes are mounted, while drilled along the centerline (credit to my good eyesight), were also drilled using a Dremel tool... hence the little 'whoopsie' with the spacing of the center dome relative to the other two. I don't have a drill press, so there's no real way around the issue other than trial and error. Everything else about the design I fully expect to improve with each attempt, but the drilling is still a concern.
On the other hand, I'm having loads of fun with this. Here's a completely irrelevant picture of my cat.