> However, the system was repaired and pneumatic clocks continued to work until 1927.
Considering that the RER started to operate in 1977 [0], I find that hard to believe.
[0]: https://en.wikipedia.org/wiki/R%C3%A9seau_Express_R%C3%A9gio...
I know in NYC that water pipes and electric tend to get routed through/around subway tunnels once constructed.
The northern terminus of the Ligne de Sceaux opened at Luxembourg in 1895. Between 1973 and 1977 it was converted into RER B.
( https://en.m.wikipedia.org/wiki/Luxembourg_station_(Paris) )
> The RER's tunnels have unusually large cross-sections. This is due to a 1961 decision to build according to a standard set by the Union Internationale des Chemins de Fer, with space for overhead catenary power supply to trains. Single-track tunnels measure 6.30 m across and double-track tunnels up to 8.70 m, meaning a cross-sectional area of up to 50 square metres, larger than that of the stations on many comparable underground rail networks.
Paris has basically three distinct stages in "urban rail" development:
- first, the mainlines were built from stations on the then-outskirts of Paris (Gare du Nord, Gare de L'Est etc.)
- then trams and later metros were built to link these train stations to the city and serve additional areas.
- finally starting from the 1970s they started building rail tunnels so suburban trains could run directly from one train station through the city to another station - similar to the S-Bahn systems in Germany or Crossrail in London. The RER tunnels were all built in this phase.