Zagreb doesn't have a metro line, so tram is the principal source of transportation in the centre of the city. Zagreb Electric Tram or ZET (Zagrebacki Elektricni Tramvaj) is the company that takes care of this. Founded in 1891, the first trams they had were drawn by horses.
There are 116 km of tram tracks in Zagreb, on which 200 trams transport more than 200 milion passengers every year. Due to different manufacturers, trams in Zagreb vary in size, shape and capacity. They were one of the first ones in the world to introduce purchase of the ticket via SMS.
One of my muggle friends suggested that I put a cache on the tram, that would be *reeeeeally* difficult to find. I decided to do exactly what he said, so here we are. 
Just come to the listing coordinates, and wait until you see the right tram number, you'll know which one is it, trust me... The area is infested with muggles, and there is a security guard across the street who often walks and looks around, so pay a special attention when retreiving and putting back the cache. You probably know this already. Good hunting!