There are 6-7 train-stations in Paris.
7 if you count Gare d'Austerlitz but the traffic there is not really important.
The train stations started with the arrival of the train during the XIX c.
We had a monarchy in that times.
King Louis Philippe the last king ruling France worked for it development helping the new private companies.
The train was a symbol of positive evolution, new technologies so it was important but it was also noisy and could be dangerous.
So the train stations were beautiful and had commodities for the upper class but were located at the limits of the cities. Like Paris.
Gare du Nord and Gare de l'Est as well as Gare Montparnasse were located almost out of the city when built.
These train-stations were sort of the new doors of the city.
Many of them had to be restored to follow the evolution of the needs of the travelers.