Between mid-July and early September, ALSA runs two or three morning buses daily from Cangas de Onís to Caín (two hours), with return service to Cangas twice or thrice daily from both Caín (two hours, return fare €14) and Poncebos (45 minutes, return fare €12.45).
On weekends during the same period ALSA runs a morning bus from León to Caín (2½ hours, via Posada de Valdeón), returning in the afternoon from Poncebos to León (3¼ hours, via Arenas de Cabrales). The return fare is €20.
From the eastern side of the Picos (the Potes area), drivers will be well rewarded by a trip around the southern approaches to the Picos. From La Vega de Liébana, the N621 southwest rises to the Puerto de San Glorio pass (1609m), where you enter Castilla y León. In clear weather the 2km detour to the panoramic Mirador de Llesba (1675m) is very worthwhile. The N621 drops down quickly on the Castilian side of the frontier.
From Portilla de la Reina, follow signs towards the Picos de Europa and take the narrow and pretty LE2703 northwest across the Puerto de Pandetrave (1562m) to Santa Marina de Valdeón and Posada de Valdeón, where you're at the southern gateway to the Picos. Caín is 9km north of Posada.