Can you explain:
* the purpose of this journey (on this busy route, far from the depot)
* what the tram and truck are doing on this track. I think this is the track entering the loop and not exiting the loop. Something looks wrong here.

BTW: I'll correct this place as "terminus Moillesulaz". MOILLESULAZ is not the standard for this site. In any case, thanks to share this interesting photo!

There was an event surrounding the future extension of line 17, which opened 3 months later. A flyer can be found at https://www.bus-tram-geneve.ch/site/fete-sans-frontiere/.

The tram itself is not technically operable and requires to be towed by some vehicle. In this case, as the event was held on the two tracks heading to/from Annemasse, the 80 was towed to the main track after the event, then pushed backwards (as shown on the picture) in order to attach it to a DAV Be 4/6. It's the most realistic hypothesis.

Thanks for this valuable and interesting explanation.
I had not taken into account the option of the tram being pushed: I assumed the most logical option of it being towed. Of course, the difference is not visible in a photo... So now I can follow the logic and the event. Thanks again for the explanation.