If we assume your consist is complete and not missing another coach then a reasonable guess would be a #617.
The reason for calling this a guess is the question of when American Flyer went from brass stampings for the car door details to decals. The 1933 catalog doesn't provide specifics with respect to brass details on the cars. The illustration above gives the impression of decal doors but that is all.
The other reason for the qualifier is, if you are missing a coach, then the 1934 catalog has the Iron Duke headed by 3309. The 1934 catalog also shows a three car set headed by #617 but the consist is an observation and two coaches.