Over the years, Greenport has hosted various events that centered on the visits of large/tall sailing ships. I did some online research, but was unable to identify a specific sailing ships event held at Greenport in October 1979.
This could be a special excursion train operated for the event, but it could also be a regularly scheduled train with some extra 2900 series P72 coaches and parlor observation 2082 ASHAROKEN added to accommodate the increased demand. It is interesting to see a custom designed tail-sign in the drumhead can; I wonder if this tail-sign was produced by a sponsoring organization or by someone at LIRR? As for motive power, I agree that a pair of MP-15 locomotives would be probable. Was the Greenport turntable still in service at that time, if they wanted to turn a GP38-2 ?