Andy Sparberg is probably correct about 1939-40 World's Fair service. I didn't recall the picture in Kramer's book being pre-opening, but I could see the photo with the 'S' in my mind's eye as I was typing. And 'S' was always 'Special,' as I mentioned in an earlier post, never 'Shuttle' (until the TA went on its binge of trying to get people to stop thinking of some trains as expresses and others as locals).
I could never understand who at the TA was so insensitive to World War II survivors living in the City as to have 'SS' trains running on the subway. They carefully avoided 'P' and 'PP' trains, apparently not to offend those squeamish about bodily functions.
Happy holidays.
Ed Alfonsin
SUNY at Potsdam