While this topic isn't one where I have much expertise, I do recall reading/seeing that the Sacramento Northern did haul freight as part of its interurban operation. How much of that was as a mixed consist I'm not sure. They certainly had electric locomotives (steeple jacks) as part of their fleet. Here is a description of "trailers" used by the SN: https://www.wplives.org/sn/trailer.html
So I guess some of this depends on how broad your definition of trolleys and interurban is. I did learn that SN had a subsidiary line, called that Oakland, Antioch and Eastern (familiar names for us Bay Area denizens) that hauled interurban freight/passengers - restoration of one of their flat cars was the subject of this: http://rypn.org/forums/viewtopic.php?f=1&t=28133 If you scroll down, you will see some pictures of this restored flat car and what I assume is a restored 'line car' once used by the Key System (East Bay trolley/interurban predecessor to BART)