When you leave the train (departing from Lausanne), take the subway under the tracks. At the top of the stairs, continue straight on along the platform. After fifty or so paces, take the flight of steps down to the left. You will soon cross a road (be careful, visibility is reduced, use the mirror) and continue going straight down the Rite-à-Joyeux.
Remember that the Lavaux vineyard is a protected and fragile site, here are some recommendations to preserve it