i have been looking forever for trains with SBB markings

Solid work. When choosing red as line color the train looks like a German regional train, gray gives you something close to Swiss wagons, green results in Polish second class wagons. I absolutely love it.

Changing the engine is fairly simple:
1. Install the train with the engine you want (Drosovilias Panorama Train in that example).
2. Install (the train with) the trailers you want (Y-Wagen).
3. Open the train installed at 1. with the in-game asset editor.
4. Change trailers to 2.
5. Make sure your new train isn't longer than allowed.

i watched it yesterday and it looks really good and i meant if its working because you changed the engine but i dont know how to that ;D

This is the first release, so I don't get what you mean with "now". Other than that: Yes, it's working but if you find any errors feel free to report them.

so is this working now ?