To do this, AT&T installed base stations in both the New Jersey- and New York-side ventilation buildings, which were built above the tunnel to act as exhausts while pumping fresh air in. These stations combine with antennae outside both ends to split the 8,000 feet into 2,000-foot sections — a length that makes the signal loss much more manageable.
Why not just use regular antennae throughout the tunnel? The immediate reason was space — AT&T was told that if they installed antennae along the walls of each of the three tunnels, they would likely be knocked off by the machines the Port Authority uses to clean the tunnel's walls. The long-term reason, though, was it would have forced a massive upkeep effort.
While this technology has quietly benefitted travelers and tri-state residents for two decades, it also has near-future implications — it’s likely going to be one of the ways that cellular service makes it into New York City’s subway system. According to Busseno, the plan is to use antennae along the straighter sections of the subway, essentially blasting the signals from platform to platform until they overlap enough to cover the whole tunnel. But that solution is too unidirectional to cover the ones that curve; that's where leaky coax comes into play.
There are lots of things in New York City that leak; most are bad. This is the rare exception.