The issue was not necessarily one of gauge, but of turning radius. Many trolley lines were built with very sharp curves, ones so sharp that conventional equipment could not stay on the rails. Trolley modelers use very sharp curves--in HO, the equivalent of 6"-8" turning radii is not uncommon.
That being said, many trolley and interurban lines also carried freight, and some even had dedicated freight belt lines. The line I model, the Sacramento Northern, had two routes around Sacramento, CA: a passenger line, that went through downtown, and a freight line, that skirted the original city limits. As the city expanded, what had been the edge of town became city streets, so freight trains went around downtown via those streets.
Until 1953 these were pulled by electric freight motors--but from 1954 until 1966, they were pulled by the diesel-electric locomotives of the SN's parent line, Western Pacific (except for a segment taken out of use with de-electrification) and, prior to that, diesels were used when extra motive power was needed. And until 1947, those same tracks were shared by city trolleys!
So, if you can theoretically put in place a shared-trackage-rights agreement with your trolley line, sure, such a setup would be possible.