Where do i start... short answer, if it was easy, it would all be done already.
Longer answer:
* We have a totally new material structure to ensure that new content can take advantage of modern techniques.
* We want backwards compatibility to ensure that 15+ years of route building isn't thrown out the window.
* We took a wide cross section of the 500,000 Trainz assets and worked on creating legacy shaders that would mimic the old look in the new engine
* One of the delays in releasing the original closed beta was working through these issue (if you are concerned now, you should have seen the build a few months ago!)
* We then got feedback from a wide cross section of the community during months of external beta testing and improved the look of a wide variety of legacy shaders as a result
* As we received problem assets, we investigated why these assets would look different to other, apparently similar, assets.
* Most often there is a different technique used somewhere in the pipeline (such as an inverted normal map) that went undiscovered in the old engine but become apparent with the new one.
* We rinsed and repeated these steps a number of times to work out "trade-offs" where missing data would be "correctly" interpolated
* Afaik, no-one reported this selection of assets until recently
* We are still working on improvements
* This is just one of the reasons we have not released the new routes yet
This should not have been let anywhere near even early access until such fundamental issues were sorted.
If it wasn't for Early Access, we wouldn't have a sufficiently wide network of users providing us with the additional feedback required.
Then having written the above, apparently the isue identified in the Skipper content is nothing to do with shaders at all, but rather a content issue. Note, saying it is a content issue does not mean "it's the content creators fault". It means that previous versions of Trainz didn't assist the content creator doing it the right way and it didn't matter if it was done the wrong way.
The problem in more detail is as follows;
* the material is double sided (i.e. the materials that appear "black")
* the normals are (as specified by the creator) the wrong way around.
* that means that the side you think is going to be brightly lit is dark, and vice versa
* in T:ANE, double-sided materials were handled incorrectly with both sides being lit the same.
* in TRS19, we handle it correctly:
** seen from the front, it gets the standard lighting applied
** seen from the back, the game automatically reverses the normals and shows you the correct lighting.
From a code perspective, there is no bug in TRS19. However, we also realise that content creators were not aware of the problem in previous versions, so we're investigating (and will implementing) a code workaround which would result in older assets being handled in a different manner that attempts to correct the normals at load time.