The Rebbe Rashab and Eruvin in Russian Cities
Eruvin in Russian towns and cities during the Rebbe Rashab's era are part of Chabad communal history. They cannot be applied to modern New York without comparing the actual city conditions.
Russian towns and cities had their own boundaries
Eruvin in the time of the Rebbe Rashab belonged to places with their own walls, gates, population, roads, and municipal form. A town might have working gates, a limited number of entrances, or a street network far smaller than modern Brooklyn.
The halachic decision applied to those facts. It should be understood as an actual local psak, not reduced to the bare statement that a Chabad community once had an eruv.
Cities change across time
Population growth, demolished walls, new rail lines, widened roads, and suburbs joined to the old city can change an eruv question even in the same named place.
A historical eruv therefore needs a date and physical context. What enclosed a city in one period may no longer surround the city that exists later.
The precedent is the use of halacha in its place
A valid earlier eruv shows the mitzvah and the practical use of hilchos eruvin. It also shows Rabbonim applying those halachos to the real community for which they were responsible.
That is the lesson relevant to New York. Crown Heights must be decided through its own streets, population, boundaries, and Beis Din, not by borrowing the conclusion reached for another city.