One minor nitpick: zooming the map is very slow (maybe Leaflet is not the best choice?). And the main station in Paris is missing: Châtelet-Les Halles.
Leaflet should easily handle stuff like this if configured correctly. OP just slaps 3000 markers in a single layer, and each of them is an image element in dom. Should probably use some marker clustering for that.
One minor nitpick: zooming the map is very slow (maybe Leaflet is not the best choice?). And the main station in Paris is missing: Châtelet-Les Halles.
Other than that, incredible work!! Amazing.