Not sure if this is a bug or a feature :)
Trolleybuses currently can pass other vehicules, as they are road vehicules. While passing is not impossible for trolleybuses, it is usually frowned upon, as the risk of losing connectivity with the catenary is too great. Should road vehicules using overhead power ignore the passing code?
I don't know all kind of trolleybus-systems but the ones we have here can not loose the connection,
in an accident they managed to pull down the electric-cables, at the end the cables were down at the street and cars but still connected to the buses current collectors :o
So they actually can overtake other vehicles and do it, unless one trolley intents to overpass another trolley - that was what caused the mentioned accident ;D
Normally trolleybus' pantograph allows passing.
Somewhere in Internet I saw trolleybus passes other trolleybus driving in same direction. First stand on passenger-stop and the other drives without stopping. It's possible through cable-crossing on overhead power line. Unfortunately, I haven't seen in Simu two passing trolleybuses.
So I think passing possibility should be leaved for trolleybuses.