If you study the history of the Paris metro, you will discover the fact that almost all the tunnels had already been dug and the rails were laid in them before the official opening of the metro in 1900. This means that already before the beginning of the 20th century there were technologies that allowed the massive laying of tunnels hundreds of kilometers long.
From the history of the Paris Metro:
“Opened on July 19, 1900 for the World Exhibition; a ticket on the first line "Château de Vincennes - Port Maillot" cost 15 centimes in old francs for the second class and 25 centimes for the first. Most of the lines in the center were built by 1920. (How it was built and why it was built so quickly is not indicated) The features are short distances between stations, high density of the metro network in the city center and the design of the first stations in the Art Nouveau style by designer Hector Guimard. throughout the 20th century, extending their lines to the suburbs. (This means only one thing: what we have been building with our technologies for decades, the departed civilization built much faster).
The Paris Metro has 14 numbered "large" lines, plus 2 short (3-bis and 7-bis) - the former branches of the 3rd and 7th lines.
Track length 220 km, 303 stations (including 62 interchange stations). Since interchange stations belong to more than one line, the total number of stops on all metro lines is 383. The average distance between stations is 562 m. (Probably, with such a density of stations, ground transportation was not used, the streets were quiet, safe and the air was much cleaner) Almost all lines are underground, there are only 21 aboveground stations. 303-21 = 282 underground stations (Wikipedia).
Next, watch a video showing the "construction" of the Paris metro.
Already finished tunnels are cleared, repaired, cars are run through them, and the metro is ready!
History of the construction of the Paris Metro:
Promotional video:
Development of the Paris Metro from 1900 to 2011:
Entry and exit from the metro in Paris 1900s: