My experience and a pet peeve of mine is that most times, when a manufacturer such as Lionel, advertises an engine as being able to run on a specific track - like O-31 - they are talking about negotiating an O-31 curve - not an O-31 switch.
In addition, when they advertise an engine as being able to run on a specific track - like O-31 - they are talking about a single engine, not necessarily an engine with rolling stock attached.