flashes,
Just been into Upton and the map shop.
The 4 maps for your mates area are all old imprints but current editions (1999-2009, Editions 4 or 5) and none of them have the new line marked.
I misread your post and thought the existing SNCF line was the one you were after. I didnt know about the SEA being constructed in parallel.
IGN have marked the new line construction on their 1:50,000, but when you zoom into the Carte Bleue 1:25,000 it disappears. Check it out here:
http://www.geoportail.gouv.fr/accueil?c=0.12547069256574317,46.019788737459756&z=0.00008583068847489914&l=GEOGRAPHICALGRIDSYSTEMS.MAPS.3D::GEOPORTAIL:OGC:WMTS==aggregate(1)&permalink=yes
zoom in one notch and you’ll see the line disappear.