In terms of jbeam, it can be quite simple, or complex, how you want, this would be 3 levels I guess: Thing is, you do mesh, then do jbeam model, making jbeam from graphical model rarely works well. To make moving parts will then add to complexity, you would do upper level from two quads instead of one, to have hinge at middle, then have two beams to act as hydraulic cylinders, these would be hydros in jbeam language, wiki has more about those. This is just poorly made 3 level mockup, but those selected edges would be hydros:
Definitely have to see a modern american freight hauler. Maybe a ES44AC(pictured above) or an SD70(pictured below)?
I doubt, it is not quite that simple to edit existing than create new one. While creating some rough beam structure takes only 10-15 minutes in Blender, one needs then to do a lot of parameter setting as well as other edits to it and currently there is no way to import beam structure to anything. NBeditor is so buggy it can only be used for viewing really, so it would be lot of manual editing in Notepad++ and that is something my abilities are quite incomplete with. Anyway, if you can model a train wagon in Blender, then making basic beam structure will be really easy, just take basic cube and scale, extrude and make few extra edges, remove not needed faces, then use jbeam exporter on that and you will have new nodes and beams. Hard part starts after that, days and weeks of editing and finetuning, part I'm not good at all.
When Khalibloo implements importer function to his Blender plugin, then it should be much more possible to edit existing jbeam: https://www.beamng.com/threads/blender-addon.56981/ That is something I really look forward to, because it would make so much more jbeam work possible for me.
In a few days maybe weeks my Russian Locomotive and Russian Wagon will appear on BeamNG fun youtube channel don't worry I gave them to him exclusively. I need some feed back. --- Post updated --- Yes, but i have some problems with engine sound... --- Post updated --- Next, I hope I will make autotrain wagon (car transport wagon)
How about you start making train cars so we can have an actual train in the game? Maybe a tanker car or box car to start out with. And we could use the coupling system in the game to connect them.