I've been reading a book on Italian Economics and it is examining why the south of the country generally has a weaker economy. It notes that despite the Kingdom of Naples being the first part of Italy to industrialise and being the location of the first railway in Italy, it's economy never took off. It blames this in part to a ban on the construction of railway tunnels which stopped the railways from becoming economically useful. It says the ban was instigated by the clergy as they saw railway tunnels as a threat to public morality. I can't find any more information on this and the book does not elaborate further. Do any of you know why the clergy thought railway tunnels were a moral threat and what their aims could have been?
It sounds like your source is conflating two different stories that happened at the same time.
Italy is pretty important to the story of railway tunnels, if nothing else for the tunnel at Mont-Cenis, the first dug using a new compressed air drill devised by three Sardinian engineers -- this started in 1861. But to get there required railways and tunnels to be acceptible in the first place.
Italy's first two railways (1839 and 1840, in Naples and Milan respectively) were tunnel-free. They also, based on the fact they essentially were meant to cater to their respective courts, not meant to democratize space. Already having been the one to greenlight the 1939 railway, the King of Naples (Ferdinand II) was given a project proposal for a railway going all the way from Naples to Avellino to Bari. The plan required the use of tunnels. The king was wary of the tunnels and that men might "touch" women while in the darkness; when told tunnels were needed to even reach Avellino (let alone Bari) he reportedly said
Ma che c'è di interessante ad Avellino?
that is, "what's interesting in Avellino?" Note that there was not a legal issue, just Ferdinand II disapproved, but also, based on context, that such a thing was not in the King's interest.
Going just a bit north to the Papal States at roughly the same time: Pope Gregory XVI called railways (not tunnels) "roads of Hell" and they were outright banned from the Papal state, with objections (as listed by the Conservatori, the counselors) as that: they will compromise security, introduce more foreign and illicit goods, and raise the amount of poverty. (Reading between the lines, there was the implication of more beggars and criminals being able to make their way to Rome.)
So to be clear, the "tunnels are immoral" came from the King of Naples, but wasn't a ban, just a general opinion, and "railways are bad" -- and a ban on railways in general -- came from the Pope.
This was incidentally not that long-lasting a standpoint; Gregory was followed by Pius IX in 1846, who felt different, and specifically thought the opposite would be true economically: he welcomed the new "railway mania". This was considered worldwide to be a throwing off an old, corrupt government and a sign of modernization; the Economist wrote:
For the first time for many ages a chord has been struck at Rome, which now vibrates from Calabria to Piedmont and through the whole of Catholic Europe.
Three railways were announced by the Secretary of State (Gizzi) in November, but the timing here was rather bad (the Revolution was right around the corner in 1848 and the Pope briefly had to go into exile) and it took until 1856 before one of the proposed railways (Rome to Frascati) finally opened.
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Bracalini, R. (2001). L'Italia prima dell'unità: 1815-1860. Italy: Biblioteca universale Rizzoli.
Schram, A. (1997). Railways and the Formation of the Italian State in the Nineteenth Century. United Kingdom: Cambridge University Press.