It is the second oldest metro in the world after London
Unlike London it was build right under the surface
Every line was owned by a different company which is why they don't share tracks and why the connections are so messy. It was only later that the French government took over.
The tunnels were deliberately created too small for the cars of the trains of that time so that the big train companies did not take over the project.
For the same reason the network was not to go outside Paris, so that the city of Paris was the only one in charge of it and to keep the project out of the hands of the government.
This system is still working, it is only once the RER trains leave Paris that the SNCF (French railway) is in charge of them.
There is one fully automated line (14) and a second one is being transformed into one (1)
There's ghost stations, closed to the public and not in use for decades, that you can visit once a year like Arsenal, Haxo, or Porte Molitor.
Stations are no more than 500 metres from another stations.
The famous iron works marking the entrances of many of the stations date from 1899.
