How do you run trains under Paris and in the Île-de-France region? RER

 https://www.youtube.com/watch?v=xylmrycsSW8 , https://www.ratp.fr/plan-rer 

https://en.wikipedia.org/wiki/Rseau_Express_Regional

A Transit System of EPIC Proportions | Paris RER https://www.youtube.com/watch?v=6afb6uv1V1Y 

https://en.wikipedia.org/wiki/R%C3%A9seau_Express_R%C3%A9gional#Rolling_stock