Is it allowed to use a sound signal in this situation?

Why?

The maintenance tram is manoeuvring in front of your vehicle, so you should wait for it to pass. In a built-up area, you may sound the horn only when necessary because of immediate danger. Having to wait for the tram does not justify honking or urging its driver to hurry.

art. 29 ust. 1 i ust. 2 pkt 2 Prawa o ruchu drogowym