Jon,
I can add to this, though most of the stuff you questioned has been answered. This is more of a real-life very much related story on this very subject. About 20 years ago I worked for a company that created training programs for the plastics industry. I did the multimedia authoring while another teammate did the 3d modeling. Keep in mind our PCs in the office were not powerful in fact pretty paltry even for those days. The fastest machine was a Pentium 90 and that was the modeling workstation. The rest of us had 486s with less than a gig of RAM and barely enough steam to get out of their own way. even when editing text in Word.
We had a big project to do. A consulting project which required imported models from various sources with one of them being some older models generated by the company owner's brother. These were some rather nicely detailed injection molding machines which were all animated to show parts being created, rods moving, and other stuff. The 3d modeler assembled these models into the scene and setup the animation. We were using 3d Studio R4 for DOS, which like the current versions allows for multiple machines to render the scene in a round-robin fashion. This means there's a control list generated by the main program which is read by the connected workstations. Each PC only needs the software installed and no dongle or key-code is needed for rendering because nothing else is enabled. Each one of these machines, with and without the dongle, read the control file and grab whatever frames are ready in sequence for rendering. This means some of the individual images were on one machine, while some on another. In the end, they are batch copied to the final rendering machine to generate the animation. This sounds a bit complex but it's not.
Anyway, we set up the machines to render and we went home, figuring that the animation would render completely by the time we got in the next day. We came in and only 20 seconds of the full 2 minute animation was rendered! What??? Huh??. I got called to look into the problem while the animator got busy checking out the models. The workstations were all working fine. The network was there, and the control file showed who had grabbed what frames. My paltry system was already at a 100:1 disadvantage being a '486, and had rendered only 10 frames out of the 20 seconds of video.
We adjusted some things, thinking we fixed the problem and went home. Got in the next day only to find the same problem! We couldn't figure it out. The boss was getting antsy over wasted time and money and I was sent off to do something else while the animator looked at the scene again. We repeated the process again, only to have the same issue. We couldn't figure out what was going on because all the models looked fine. Hmmm... We thought. Those machines look so great, but maybe we should replace those just because, besides they were the only things left in the scene that hadn't been touched.
The animator pulled those from the scene and we tried a test animation. What had taken all night to render 600 frames of animation rendered in about an hour. Hey what's going on here... He pulled the machines apart in the modeler and found that the original guy, the owner's brother, had put nine beautifully very, very high polygon feet on the machine. Each foot was set with the highest amount of polygons capable of the software. This most polygons any of us had ever seen, let alone on something that was never seen in the video. After removing those feet, the animation rendered in a few hours overnight.
Sure things aren't always this extreme, but it shows how much smoothing can affect a model. For the most part, the Trainz modelers do a good job optimizing their models so they look good, yet aren't so overly built, like the machine feet, that the game comes to a complete stop. Even today this can become a problem and it something to keep in mind should you ever come across some really bad performance on a route. Go through and replace the assets in that area and see if that helps. It could be one of the objects is a bit heavy and dragging the system down.
John