Thanks, Kris.
Indeed, I didn't find any graphic with the Clinchfield F-unit, which is used in the current menu background.
Suppose the menus are in those .swf (Flash player) files from railworks\data\scaleform. I believe to remember to have edited RS_2013_MenuSystemMaster.swf in an older incarnation of the game with a .swf editor to get my custom background in the main menu.
Success! By using
https://github.com/jindrapetrik/jpexs-decompiler and the latest Java runtime I was able to change the #311 graphic, which resembles the main menu background.
My UK install of the game now looks like this:
Clipboard02.jpg
Some fine tuning of the picture is still needed. But, in principle, one can add his own menu background.
In fact, one can edit all interface elements this way.
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