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Probably never. The tunnels cost a lot to build but, once built run almost for free - they're powered by gravity and will keep running for close to a century before major maintained is needed.

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Yeah that makes sense but if growth dictates another tunnel... And it takes another 60 years, your capital expense starts to look a lot like an operating expense. Not to mention one of the big stated purposes of this tunnel is actually to facilitate maintenance of the other tunnels. There is probably more operation cost hidden here than seems obvious.

The big reason for tunnel 3 isn't new population growth, it's so that the other tunnels can be shut down for maintenance and inspection. NYC's population is more or less stable over the last 90 years.

Close to a century?

There are Roman aqueducts in continuous operation for two millenia.

AFAIK there is one still in operation and entirely for tourist purposes

Today as in Ancient Rome the aqueduct terminates at fountains. It does not connect to the standard modern municipal plumbing. Whether that counts as being operated for "tourist purposes" is a matter of definition.

https://en.wikipedia.org/wiki/Acqua_Vergine

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