Now it is another file:

.../railworks/data/textures/frontend/2014_BG.tga

The new loading screen for the routes is:
.../railworks/data/scaleform/RSSlideshow.swf - now, this must be a flash file - not a jpg.

You can download my loading/background files:

TS2014-loading-screens_by_newS.rar

unpacking into the folder:
... :/program/steamapps/common/railworks/