Hmmm... why would you say that there is little economic sense? You are a smart man which is why I try to understand your point of view before discarding it.

So do you believe that the way it was designed (its limitation to Gare de l'Est? the way it was going to be financed? independent from other rail connections?) makes it economically unsensical? Or do you actually think that fast rail links between city centres and airports are non-sensical?

I don't know about for instance Heathrow Express and its business case, which is the most comparable case: big airport, big city, bad traffic on roads for big parts of the day, service limited to one mainline station in one part of the town, service in parallel to a dirt-cheap (or rather "cheap but dirty") option (RER in Paris, Tube in London). Has it paid off?

Personally I would be a potential user of a CDG Express at certain hours of the day if it is well done, i.e. if it is easily accessible from all terminals, if it is reliable, clean, air conditioned, and especially if it is really fast. Just like I use HEX. It would be a great alternative to sitting in the back of a taxi for an hour. But then "creber likes it" doesn't mean it's a great business plan

The other thing I just thought is how Paris is really the capital of making transportation more complicated (closing down la voie sur berges - can one be more idiotic?) and of missing opportunities to make it better. Over the years other cities have built fast train links, reliable and comfortable public transportation, intelligent road management and road pricing systems - all we have is a bike shed every 150 meters. I love them, but whoever believes that this is the answer to the menace of transportation collapse is delusional.