Voie 1 (normal route): K 1-2 easy.
Voie 2 (southwest ridge): K 2-3 fairly difficult.
Voie 3 (west wall): K 4-5 very difficult.
Take the Montreux-Oberland-Bahn MOB railway from Zweisimmen or Montreux to Rougemont (992 m), then a few minutes' walk down the road to the Saane river (French name: Sarine) and to the valley station of the gondola cable-car to Videmanette (2,160 m); this operates daily from mid-June to mid-September; at weekends until mid-October.
From the cable-car mountain station down into a gap (2,121 m).
The easy Voie 1 and the moderately difficult Voie 2 both lead over the bottom part of the south-west ridge and then make towards the south wall. After a rock slab, the routes separate: Voie 1, used mainly in the descent, climbs through grassland directly to the main peak of Rubli (2,285 m); there is a cross at the top, and a book to record the names of climbers. Voie 2 climbs the south-west ridge up to the western peak, from where a ridge path runs to the main peak. The difficult to very difficult Voie 3 across the west wall drops briefly from the gap as it heads to the north, crossing through a waterfall chimney to the impressive start of the via ferrata. After the overhang at the end, you gently ease yourself onto the top of the western peak.
Using Voie 1.
Eugen E. Hüsler/Daniel Anker: Wandern vertikal. Die Klettersteige der Schweiz, AT Verlag, Baden 2004
Region: The Alps of Vaud, Pays d'Enhaut, Rougemont
Peak: Le Rubli (2,285 m).
Time required: depending on the choice of route, 1 1/2 to 2 1/2 hours.
Variation in height: around 200 m climb.
Lots of fun! But the Voie 1, according to this site: http://www.chateau-doex.ch/en/sports_en/sports_activites/sports_ete/marche/ferrata Is also marked as level K 2-3. And I believe it is correct.