Can't speak for other systems, but Pacific Electric had freight hauling operations. During the early days, they actually serviced part of the Port of Los Angeles and owned 25% of the Harbor Belt Line along with Southern Pacific, Union Pacific and Santa Fe. I've never seen a photo of a passenger unit coupled to anything other than another passenger unit and as I recall, the couplers weren't compatible with a freight car anyway. Add to that the fact many of the interurbans had wood beam frames that would make them incompatible with hauling freight. PE had electric locomotives handling freight cars. I recall seeing a photo of a small "freight motor" that was basically close to a motorized box car, but I can't remember if it was for actual small, local freight delivery or part of the maintenance fleet.