I've tried searching around (in English) for people with this theory: I only managed to find one or two results that mentioned this when searching either for
spirited away train station girl or
spirited away train station setsuko.
That aside, the girl at the station doesn't obviously look like Setsuko. Setsuko generally looks rather small and (slightly) chubby, in a very childish way. (After all, Setsuko is only four-years-old or so.) This is the case even at the end of the film, where Seita and Setsuko look over Kobe in spirit form: the characters look the same age as they were at their deaths.
Meanwhile, the girl at the train station is somewhat taller and older in appearance. This girl also doesn't have Setsuko's hair (at least not overtly) and isn't wearing the sort of clothes we'd usually associate with Setsuko.
Absent such obvious markers, fan commentary, or any statements (e.g. in the form of art-book trivia) by the producers, I would conclude that this most likely isn't Setsuko.