The T13 tram-train connects Saint-Germain-en-Laye to Saint-Cyr-l'École, also serving the cities of Mareil-Marly, L'Étang-la-Ville, Noisy-le-Roi, Bailly and Versailles.
The project is based on an existing line: the Grande Ceinture Ouest (GCO). It runs on the national rail network as well as in the city.

