The poskim have already called attention to the fact that the Mishnah Berurah had obviously not seen the Bais Ephraim (Toldos Shmuel, 3:81:7, 3:86:8; Bais Av, 2:5:2; Divrei Yatziv 2:173:1, and Even Yisroel, 8:36). We can add that this is evident from the Mishnah Berurah himself since he states that he did not possess the sefer Bais Ephraim (Bi’ur Halachah, 208:9, s.v. Eino M’Vorech). The poskim postulate that had the Mishnah Berurah seen the Bais Ephraim he would have paskened like him that shishim ribo is an accepted fundament of a reshus harabbim, and he would have agreed that even a Baal Nefesh could be lenient and rely on the fact that the streets are lacking shishim ribo and that regarding mechitzos we pasken lo asu rabbim u’mevatlei mechitzta. In any case, we now know that the Mishkenos Yaakov’s assertions, which the Mishnah Berurah upheld, that most Rishonim pasken that shishim ribo is not a criterion of a reshus harabbim and that most Rishonim maintain asu rabbim u’mevatlei mechitzta is incorrect. There is no doubt that the Bais Ephraim’s assertion that the overwhelming majority of Rishonim pasken that shishim ribo is a criterion of a reshus harabbim and that most Rishonim maintain lo asu rabbim u’mevatlei mechitzta is correct.
Notwithstanding the Mishnah Berurah’s opinion that a Baal Nefesh should be stringent regarding both shishim ribo and lo asu rabbim, there is one criterion of a reshus harabbim that would allow us to establish a city eruv today in such a manner that the Mishnah Berurah would agree that a Baal Nefesh can utilize it. The Mishnah Berurah (364:8), when describing the cities of his times, stated that there were streets that were sixteen amos wide and mefulash m’shaar l’shaar. Therefore, a Baal Nefesh should be stringent since to erect an eruv in these cities they had to rely on the fact that the street did not have shishim ribo traversing it. (It is important to note, like the Magen Avraham and most poskim, the Mishnah Berurah [345:20] understood mefulash m’shaar l’shaar as meaning mefulash u’mechuvanim m’shaar l’shaar, open and running straight from city gate to city gate; see Mefulash According to Most Poskim.) As we know that most towns in the Mishnah Berurah's times were not walled ― even in earlier times most cities were not walled, Pri Megadim (Mishbetzes Zahav 362:17) ― we can deduce that the Mishnah Berurah accepted the criterion of mefulash u’mechuvanim as not being conditional of a walled city. Since there are not many streets in a large city that would be considered mefulash u’mechuvanim m’shaar l’shaar this fundament of reshus harabbim is lacking and the Mishnah Berurah would agree that even a Baal Nefesh can rely on an eruv of tzuras hapesachim. (Many towns in the Mishnah Berurah’s era had streets that were mefulash u’mechuvanim m’shaar l’shaar. As Rav Shlomo Dovid Kahane zt”l declared [Divrei Menachem, O.C. vol. 2, pp. 42-43] a small town would have a greater problem establishing an eruv since its streets would be mefulash u’mechuvanim m’shaar l’shaar. In a small city there is usually one main street running straight through the center of the town as opposed to a large city where the streets are generally not straight from city gate to city gate.)