Из порта? По какому маршруту он добирается в Хёмниц?

We (the author and I) also don't know from where this tram came.
We only know that this was the start of the transfer AND that no tracks are behind the harbour. This train started in Antwerp (harbour left side) and goes to Montzen: this is the Belgium / German boarder for freight trains.

Would it be possible to transfer trams on train rails in other countries than Germany and Belgium?

Becase AFAIK in Czechia there is strict division of trams and trains, which is pity, because for Ostrava it would be great.

The vehicle has 2 numbers (both in the database), including a DB number. This means that this vehicle is allowed to run on railway tracks and this on a normal speed.

I believe this vehicle can be transported anywhere in Europe.
Independent driving will only be permitted on routes where this is expressly authorised, as is the case for all rail vehicles.