Unfortunately, conversion of 'stock' models (both sketchup, and 3D model stock stores/sites) is rarely as simple as grabbing a 3D model and then exporting it.
For good, game ready, models you will often only need to make LOD and possibly reduce texture size (as many these days use 4k, 8k, or greater textures specifically so you can alter them to suit the game engine they'll be used in), and configure material names. Even then, conversion of these objects still requires some work as you absolutely need LOD on them and there's often minor issues that need tidying up for use in specific games.
But there's a great many poorly created stock models (as far as game use is concerned, the model itself could be extremely good for static renders) available that have either far too many polys or many separate textures. These are the ones that require the most care when intending to convert to Trainz, as at a minimum you'll need to lower the poly count for the highest LOD, and try to compile the separate textures into one or two texture maps. These assets are going to be less of a time saving, but there can still be some as you already have at least a portion of the work done for you.
That said, for 'hero' objects sometimes you can make exceptions. If for example you are building an industry, and you find an object that fits really well for the major part of the industry, you may find that a less efficient but visually spectacular object works well. But even then you need to be careful of affecting performance if you have a lot of other objects around it. Note that in this case, my reference for hero objects are those that dominate the scene (ie it is both quite large, and it is important to the scene).
I've had a fair hand in the conversion of stock models to Trainz of late (you can find some in the new Bairnsdale to Orbost route, mostly cliff splines, rock objects, and street lights around the station yards; plus a nice selection in the newer main menu routes), and there is a lot of work to do it properly, even with really good models. Getting the balance between performance and visual quality isn't as easy as many think. But one of the biggest parts of it was definitely keeping textures to a minimum where possible, and ensuring that the LODs were as effective as possible.
We do hope to one day introduce some form of content evaluation tool, but until then it's very much worth viewing assets you aren't sure about in the 'preview asset' tool and see if A) it looks reasonable, and B) that both the poly and texture count go down at the further LOD distances.
It must be noted that objects that only have one actual LOD (ie the highest LOD, and then at an arbitrary distance make the object disappear) aren't going to help performance at all. It's a valid method for testing an object, and creators absolutely should make the object 'disappear' at a suitable distance in their LOD. But between 0m and the distance it disappears at, there absolutely should be at least 2 or 3 LODs.
The exceptions of course will be very low poly very small objects that don't get a huge amount of use in a scene (ie a simple wall poster won't have a lot of polys, so LOD will be minimal at best). Remember that you need to think both of the poly count of one of the object, and how often that object is likely to be seen in the scene. For example a small-mid size rock might have only 300 polys. But it might be used 500 times in the scene (remembering Trainz can display up to 15km from the player). If you have no LOD on this, then effectively that asset is displaying 60,000 triangles.
The 'performance analysis' in the Preview Asset tool is somewhat handy here.
It places objects about every 20m (I think it was) in a grid, going off for quite a distance, with approximately 360 visible at any one time.
Testing with a 294 poly rock, without LOD I have 99372 triangles on the screen. With 5 LODs (LOD0, LOD1, LOD2, LOD3, LOD4) with transitions at 20m, 50m, 150m, 1000m, and 10000 (where it fully disappears), there's only 8256 polys visible on the screen. Apart from simply saving over 90k polys in the scene, if you think of this over all repeated objects in the scene, it can give you a greater leeway for using more or other objects.
Regards
