Christmas tram is appropriate. Because not everyone understands German this is what shows the display.
What I find remarkable, is that the tram can run without any problems.
After all, there is a fair amount of technology and electronics on the roof.
https://transphoto.org/photo/1042040/?vid=29999
The photo is not only beautiful (and actually a top photo) but also interesting, because it shows that the equipment on the roof is much better protected than it appears at first sight. I had rather expected short circuits / all sorts of computer errors.
The tyre sensor of my car is clearly more sensitive: 5 cm of snow was enough to generate error codes. Maybe that's why I'm surprised that a tram with so many electronics on the roof runs without a problem...

Thank you for your comment. The problem wasn't all the snow on the roof, but the snow and ice in the grooved rails. The vehicle electronics are unfortunately very susceptible to light snowfall, but for some years there has also been a special drive switch that bypasses the electronics up to 5 km / h, making it possible to start off.

Quote (R.D., 28.02.2021):
> The vehicle electronics are unfortunately very susceptible to light snowfall,

A common problem with low-floor trams.