> Having more than one railway terminals for a city as big as Paris is not unreasonable.
> There are technical difficulty with connecting the stations
Both points of yours are right, but something is missing in the picture.
Several non-connected terminals are the right solution is you only consider the service to the people living in the city, but if such city also acts as a transportation hub for a greater region, then you must ensure that travelling via the hub be possible without having to load your luggage on a crowded bus.
> There are technical difficulty with connecting the stations
Both points of yours are right, but something is missing in the picture.
Several non-connected terminals are the right solution is you only consider the service to the people living in the city, but if such city also acts as a transportation hub for a greater region, then you must ensure that travelling via the hub be possible without having to load your luggage on a crowded bus.