BTW, in case you're keeping track, the Hart convertible gondolas were the fourth kind of freight car I printed on the Form One. I've put together a list of all the 3d printed cars I've made, and mention some brief details about each.
With the Hart convertible gondolas, I've finally learned the dirty secret of 3d printing.
Now, if you scratch build a model, it's really easy to make one. You build it slowly; if you get stuck or if something doesn't work, you scrape off the part that isn't working and remake it. Eventually you've got a very nice model. You've also spent a fair amount of time to get to that point. You might be unwilling to make a second, or third, or fourth car because, hey, that would be boring. Making cars only as long as they're fun is fine if you only need one of a particular car, but not so good if you really need a dozen identical cars.
Now, if you were designing an injection-molded freight car, you're definitely aiming at production. You're making either thousands of models for yourself or for others. You'll put a lot of thought into the design because cutting those molds is time-consuming and expensive.
With 3d printing or resin casting, you're in the middle. You're making a small number of items, but you're having to figure out what to make. 3d printing make it easy to sketch up a first model, print it, and iterate. And as a result, you end up with a mighty large scrap box containing all the rejected pieces. Some might be fit only to give you a lesson about things not to do. Other rejected models might be great for a "wreck" at the bottom of a canyon. and some of the rejects aren't so bad...
And now you're in a race. Each time you make a test print, you've made a model that might almost be good enough to use on your layout. You've got some maximum number of models you can probably use on your layout. You've also got ideas on how to tweak each design so that it's closer to perfect and might be useful to other people. So the question is: how fast will you get to "good enough" models and lose interest in printing additional models?
I'm up to twelve or thirteen Hart gondolas at this point, and that's an awful lot for my layout that can't handle more than a hundred freight cars. I've got four cars modeling the gondolas with the bottom doors shut; they look pretty good, but the side sills are too short and won't fit the 7" lettering for reporting marks. So I'm caught - do I use these cars even though they're not quite right, or do I print another four or five cars to get perfection?