The only time conversation has to be restricted to subjects directly related to the movement of the train in when there is a (on the Union Pacific) "cab red zone". This includes things like approaching the end of your authority, approaching a signal that requires the train to stop or operating at restricted speed etc. Other than these types of scenarios there is no restriction on what can or cannot be discussed in the cab. I'd tell that little weazel foamer tweerp to take a long walk off a short pier (obviously I wouldn't use that language, in reality I would use much worse but this is a family board).
Scott