Why doesn't the route of the T13 tram extension only follow the tracks of the Grande Ceinture?

The route of the extension of the T13 Tram between Saint-Germain-en-Laye and Achères initially planned used the tracks of the Grande Ceinture to the entrance to Achères. After the 2014 public inquiry, it was modified to pass through the centre of Poissy and offer a connection with the extended RER line at Poissy RER station. In fact, the route of the T13 Tram extension will use both:

• The tracks of the Grande Ceinture (National Rail Network)
• Lanes dedicated only to trams in urban areas

In addition, the section of the Grande Ceinture concerned by the passage of the extension of the T13 Tram will be closed to the circulation of other trains to accommodate only the future T13 Tram. This will allow existing tracks to be used rather than having to rebuild them alongside existing tracks.