Elevated Train Tracks And Eruvin
About 50 years ago, Rabbi Moshe Feinstein zt”l (Igros Moshe, O.C. 1:138) was asked to voice his opinion on creating an eruv in Brooklyn. At that time, Rabbi Rafael Ber Weismandel, zt”l, had written that doing so is possible since Brooklyn is surrounded on three sides by man-made walls that hug the ocean and the river, and the fourth side is closed off by elevated train tracks. According to the sugya of pi tikra, the edge of a roof can be considered like a wall, which descends to close off a reshus hayachid. Rabbi Weismandel ruled that the train tracks formed such a wall.
Rabbi Feinstein responded with a lengthy teshuvah in which he rejected the proposal. One of his arguments was based on our sugya, in which we find a machlokes over when the Halacha L’Moshe M’Sinai of pi tikra applies. According to the halachically accepted opinion found in the Rema (O.C. 361:2), pi tikra applies only if there are already two solid walls with a common corner that form an “L” shape. Pi tikra can then form a third wall. However, if the two solid walls are parallel, such that people can freely pass between them, pi tikra is ineffective. Therefore, since there is nothing to stop people from freely passing beneath the train tracks in Brooklyn, the Halacha L’Moshe M’Sinai of pi tikra does not apply.
Only Beneath the Tracks
Furthermore, argues Rabbi Feinstein, pi tikra is only relevant to the area beneath the roof. Pi tikra allows us to make an imaginary wall to enclose the area beneath the roof. In other words, the area beneath the train tracks may very well be a reshus hayachid, but the pi tikra of the tracks would not be able to serve as a wall for the rest of Brooklyn.
We find this argument presented in our sugya by Rava, who claims that if a sukkah is built next to a canopy, we cannot apply pi tikra to the edge of the canopy to form a wall for the sukkah. Pi tikra can only form a wall for the area beneath the canopy, not for the area of the sukkah. For these and other reasons, Rabbi Feinstein concluded that Brooklyn cannot be considered a reshus hayachid based on the train tracks forming a pi tikra.
Bypassing a Municipal Directive
Today we find that numerous authorities have supported and established eruvin in Brooklyn and similar localities. Many take their cue from an interesting ruling of the Chazon Ish (O.C. 79:1). In a certain city, the government did not permit the Jews to build a tzuros hapesach over the main street. The street was wider than ten amos, so a lechi or kora would have been ineffective. Therefore a balcony extending over part of the street was built so that less than ten amos separated the edge of the balcony from the opposite side of the street. The Chazon Ish ruled that the edge of the balcony was considered a pi tikra forming an imaginary wall which divided the street in half. The remaining half of the street was less than ten amos, and a lechi was then sufficient to permit carrying.
Apparently, the Chazon Ish considered the pi tikra of the balcony a valid mechitzah even in regard to the street beyond the balcony and even though people passed freely underneath.