With built ins they normally wont allow you to clone them however I just tried this in TS10 and found you can clone them
However the clone wont work or the contents wont show up as it is still using the built in textures.
If you still want to put your own thumbnail then follow my tut
Tut.
locate the built in you want to change and make sure you can see it in the main list.
Left click to highlight it then right click and select open for edit in explorer you should now get a folder open with the contents .
On the left hand side away from the main list you should be able to see the folder for the item you need to copy and paste this to your desktop or to my documents.
Once you have done this you can revert and close the original file, we will now be working on the copy you now have.
You will need PEVsoft Images 2TGA open P-soft click onto File and open then browse to the saved folder in Desktop\dkcrossing gate _long and select one of the texture files left click and select open you should now see the texture file click onto file and select save as it will now save as a tga and a text.doc will be made.
Do the same on the second texture. once done you can remove the old texture files next you need a thumbnail = size 240x180 as you current copy does not have one so you may have to run the sim and place one of the crossings in surveyor and then take a screen shot = printSC you can then go to program flles/ userdata/screenshots locate the screen shot and open it resize it and save as jpg.
Next you need to put in to the config a thumbnail listing =
thumbnails {
0 {
image "Name of jpg.jpg"
width 240
height 180
}
}
Finally because this is still a copy you will need to change the kuid ID to yours, open the config and locate the kuid number let us say its this one <kuid:56789:30066>
you would need to change the kuid Id number = in this case 56789 to yours say 1234 so you would end up with kuid:1234:30066 if its a kuid 2 version like kuid2:56789:30066:1 you would once more just change the kuid ID number and end up with kuid2:1234:30066:1 close and save the changes and make sure you have put a copy of the thumbnail in the folder containing the texture files close folder making sure you have saved all the changes.
now import the copy to your content manager = file import content and make sure it is committed
You should now have your own copy with your own thumbnail
Here is a screen shot of the folder I copied