2k Curvy Cloud Sky image. Derived work from public domain sky photos. See license text for usage info. TLDR of the license text:
No modifications, no using in routes or sessions distributed over jointedrail.com, rypi.com, thebackshops.com or join-together.de, and no using in payware marketed outside store.trainzportal.com and store.steampowered.com without contacting the original author and asking for permission.
If you want to be able to do whatever go on a lookout for the texture parts and make all the Trainz adaption work yourself.