The first trolleybus route in Trieste, “la linea dei colli” (the hills route), was inaugurated in 1935 between the central Goldoni Square and Campo Marzio, a neighborhood in the south of the city near the new commercial port.

In 1940 these routes were operational: In the early 1950s, under the Allied Military Government of Free Territory of Trieste (1945-1954), the municipal transportation authority (A.C.E.G.A.T.)

More over, in 1952 the interurban line Trieste-Muggia began operation (Muggia is a little town just in the border with the "B Zone", at that time controlled by Yugoslavia).

The line was over 6 miles long, and some of it was in a picturesque scenery next to the meridional shore of the Trieste's Gulf.

The last trolleybus ran in Trieste in 1975 on route 19 (Stazione Centrale - Via Flavia).