I was surprised to discover that pak128 trams include some vehicles that can carry mail, piece goods, and cooled goods. They are named Fretadis_X_Y, where X is "postal", "goods", or "frigo" depending upon the freight type, and Y is "short" or "long", and all have intro dates of Feb 2014. The postal versions show up in the Electrics tab of the Depot. The other long versions show up in the Locomotives tab, while the other short versions show up in the Cars tab.
They always display red in the depot window, regardless of whether I choose "Append" or "Put in Front", and cannot be selected.
Eventually I figured out how to create convoys containing such vehicles, by searching for similarly named vehicles with similar intro dates.
I had to create a Citadis402_Tours_Front. Only then could I append one of the Fretadis_*_long vehicles. For the goods and frigo vehicles, this automatically resulted in the addition of the corresponding short vehicle. For the postal version, it works the other way around - the short vehicle can only be chosen when a long one is already part of the convoy, and causes another long one to automatically be added behind it. Then I can append a Citadis402_Tours_Back.
The Citadis Front and Back vehicles can also have Citadis402_Tours_suspended passenger vehicles between them, alternating with Citadis402_Tours_motor vehicles if there's more than one suspended, but every convoy can only have one freight type.
Why couldn't I just select one of the Fretadis vehicles, and then have depot window clue me in to what can be placed before and after them, or better yet, add the required vehicles automatically?
It's possible that in the game you are playing, the Fretadis vehicles have specific requirements or dependencies that need to be met in order to be selected or placed in a convoy.