Good Morning All
In regards to editing of DLC (and some built-in) content, this has actually been a case for quite some time. Some content included with TRS2006 was uneditable (although there were ways to get around this), and of course none of the built-in content in TRS2004 was editable...
We have made DLC content uneditable, both to ensure that the content is MP compatible (do you want to have to re-download a couple of GB every time you go to use the MP session because you edited one of the assets in the pack
), and to ensure that the DLC content is secure. We have, unfortunately, had many issues recently with people uploading built-in content, and in a few cases DLC content, to the Download Station or other locations. This is not something that we wish to see happen, as it hurts the creator, and in turn the community if the creator decides to simply no longer release their work.
That said, if you wish to edit one of the assets for personal use, you can simply contact the author. They may actually take your changes/suggestions onboard (e.g. more accurate physics, or fixing errors in the route...), and implement them in an update to the DLC pack... Or, if you wish to repaint the asset (not something we've ever supported with DLC's, and this was actively blocked with the Blue Comet pack - however we did not keep this on later DLC packs as there were ways to work around it at the time), they may supply the necessary files.
However we do not support the editing of DLC packs, nor have we ever advertised this. Actually, we've never even suggested that it was appropriate to edit payware content... Most groups also actively request obtaining permission before showing screenshots of reskins.
Regards