Historical City Eruvin and Modern Population Growth
A historical city eruv belongs to the walls, streets, population, and psak of its own period. When a city expands beyond its walls, opens its gates, or reaches a population never contemplated by the earlier arrangement, the familiar city name no longer describes the same halachic facts.
A city does not remain physically fixed
Walls are removed, gates stop operating, suburbs join the street network, and traffic shifts to new routes. A historical precedent must identify which facts made the earlier eruv possible.
Even when the city keeps the same name, the area enclosed and the public using it may be entirely different a generation later.
Warsaw
Warsaw is often cited as proof that a very large city can possess an eruv. The useful comparison requires a date: its period above shishim ribo before the war was relatively short, while its walls, gates, urban extent, and population changed sharply.
One must know which area the eruv enclosed and whether the same boundary remained accepted as the city grew. “Warsaw had an eruv” does not answer those questions.
Yerushalayim
The walled Old City and modern Yerushalayim are not one physical enclosure. New neighborhoods and public roads extend far beyond the old walls, and the city's population is vastly larger than in earlier periods.
In the recorded meeting with Israel's Chief Rabbis, Rabbi Mordechai Eliyahu discusses acquisition for the eruv, the Old City walls, shishim ribo, and the curved streets of the newer city. The Rebbe's brief lo mefulash interjection relates to that Yerushalayim discussion; the recording does not say that Crown Heights shares the same conditions.
A precedent needs its physical context
The useful question is not merely whether a city once had an eruv. One must know the date, enclosed area, walls and gates, population, public roads, and the Rabbonim who accepted it.
Only then can the earlier case be compared with a modern city. A familiar name or a broad population figure cannot substitute for that context.