This is most likely the problem(replication of what would cause said error of the tag '"'):
You can see a " in between kind and script, and to the right of kind is traincar. I'm willing to bet that is the problem. Simply remove the " and it should be fixed.
That error really just means you added an extra " somewhere near "traincar"
As for adding your own thumbnail, I find the simplest way to do it and have it work:
First paste in a picture just here
Then just replace both image sections with it, as shown below.
Realistically, it's inefficient, but I find it it to work 100% of the time - as long as I spell it right...
I believe the error about mesh_art not being allowed in container type '0' is something about that line of text is just in the wrong place, but not totally sure.
Also, what is the exact asset being installed here with these errors?