Nobody else caught on to the "its booming economy provided the financing." bit?
To say a tunnel is capital intensive is an understatement. So you need rule of law, stable and very large financial markets, relatively low corruption.
In comparison, there are other large cities built on good tunneling dirt that are perfectly unsuitable for a tunneling project for purely human reasons.
Manhattan is no picnic to dig a tunnel thru, but finances being what they are, it also looks like swiss cheese.
In London the easy bits of the 'swiss cheese' were tunnelled through first. Hence North London where the easy clay is in greater abundance has the earlier and more extensive tube trains (and more expensive house prices).
South of the river, e.g. to Brixton, took a lot longer to arrive and the tunnel work was a lot harder with former rivers and earth more substantial than clay to get through.
So yes, anything is possible given money, however, geology still gets in the way.
I've been looking at economics as a discipline, and what it does and doesn't take into account. Three of those factors are stability (which allows complex and advanced concepts to flourish, including advanced infrastructure), risk (and who bears it), and power relationships (again, who has power over whom).
Another example would be religious catacombs -- underground structures with religious significance, notably in Rome, Paris, London, and elsewhere. Again, the Church (large, stable, wealthy) established these.
Another aspect of London's underground infrastructure is its sewerage system, constructed in the 1850s, and one of the modern world's first sanitation systems. It was constructed in the face of a massive cholera epidemic (imported from India) which was claiming millions of lives throughout Europe and tens of thousands in England, and exhibiting a mortality rate of 10-20%, often killing within 12 hours. The mechanism (oral ingestion, gut development, fecal-oral transmission) had just been posited and demonstrated by John Snow and his famous map of cases identifying the Broad Street pump, contaminated by a nearby (3 foot distant) septic tank which was leaking into it.
There are many histories of this, one of the better ones I've encountered is James Burke's "The Day the Universe Changed", episode 7, "What the Doctor Ordered". The entire programme covers the emergence of the modern medical and public health system. The portion specific to the cholera epidemic and the infrastructure effects starts at 19 minutes in:
The fascinating part is that much of what's now considered to be "British nature" (cricket, stiff upper lip, jolly good) emerged as a consequence of this epidemic as well.
The Church had almost nothing to do with the catacombs in Paris (I don't know about the others, but IIRC in Rome it's also more complicated since they were created at a time where the Church had almost no political power and not very much wealth... but I could be mistaken for Rome). The Parisian "catacombs" were created in preexisting tunnels. And actually weren't catacombs at all, because people never wanted to be buried there, it's not a necropolis. The catacombs were created because the monarchy in France was pissed with the old Innocent Cemetery which was in the very center of Paris, just north of the City Island and was spreading diseases, wasting real estate and serving as a harbor to criminals etc. Also the graveyard was literally overflowing (yep, literally, basement walls in houses nearby had started to crumble under the pressure of the accumulated bones). So by a royal edict of 1785 the bones in the Innocent Cemetery where moved to an underground ossuary, that became known as the Catacombs of Paris. The Church only did the praying and rituals allowing for the transfer of the dead.
But the tunnels where actually the old Parisian quarries.. And people dug those for centuries, at times of war and peace, with rules or without. It was everything but based on a large, stable and wealthy institution. Damn, even during the Revolution people still worked in those tunnels. And miners who worked in the 16th and 17th century were pretty much free workers who lived by selling building material that they mined underground.
There's power and wealth, and there's stability, of purpose and project if not of institutions (taking your point on Paris under advisement). Simply existing for a long time (and having a long-term vision) will get you some of the way there. What modern times offers is the ability to complete massive undertakings quickly by devoting vast resources to them all at once. What ancient times had was fewer resources but often a much, much longer time horizon. What current projects are undertaken with even a 10-20 year time horizon, let alone several centuries?
To say a tunnel is capital intensive is an understatement. So you need rule of law, stable and very large financial markets, relatively low corruption.
In comparison, there are other large cities built on good tunneling dirt that are perfectly unsuitable for a tunneling project for purely human reasons.
Manhattan is no picnic to dig a tunnel thru, but finances being what they are, it also looks like swiss cheese.