Miss L. Toe wrote:

Is this a new line ?

Yup.

Entirely automatic, with the ability to stand at the front of the train
where the driver used to be. Also suicide is impossible on that linebecause
there's a wall on the platform with doors which open when the train is in
the station.

There doesn't seem to be maps on
http://www.ratp.fr/ParisVisite/Eng/Pla_q/f_pla.htm

It's there. It's the purple one from the Madeleine to Bibliothèque F.
Mitterand

The map does fail to show it now goes from the Madeleine to St. Lazare.

Also is there any benefit/difference between taking the roissybus vs the
RER from Charles de Gaulle to the centre of Paris. The bus seems a
fraction cheaper.

Depends on your taste, final destination, time of day (traffic)...

Bernard Higonnet