Despite having been on the Forums for six years (and having used Trainz for seven), I have only been on PC for four. And I'm as lost for content creation as ever.
Welcome to the club. Creating models (e.g. rolling stock, vegetation, buildings, etc) is a "dark art" but, in my opinion at least, it has always been the case. I can manage to do a reskin, a non-environmental texture or even a driver icon, simple stuff that does not involve GMax or Blender.
It just feels like there's way too much that needs to go into even a decent model, and while yes, technology gets better, technology gets more complex, I feel the bar at this point is so high it's difficult to make a beginner's guide without getting lost.
Unfortunately the trend to greater complexity has been largely consumer driven. Few of us (perhaps none of us) would be happy today using models created for the earliest versions of Trainz. Now we want our models to be more realistic looking. That means PBR textures baked in, LODs to reduce the overheads, seasonal variations (winter, summer, etc) included, etc. A lot more time, effort and expertise is now required.
As with a lot of things in life, we now expect professional results but at an amateur price.
Will I keep Trainz? Absolutely. It's the only good train sim that works on Mac, and just as a driving experience it's pretty good.
In my opinion it is also the best train sim for PCs as well. As for creating content, routes and sessions are now my forte. As long as they are still "do-able" for us amateurs I will be happy to leave the models to those who have the time and expertise.