After leaving Santander you will reach the Rio Pas at Boo de Piélagos. You can follow the riverbank to the next bridge at Arce to Oruña -which is a roughly 10km detour or use the FEVE narrow gauge train. The two stops Boo and Mogro are just 2 minutes on the train (short before and after the bridge). Tickets can be purchased at the vending machine at the platform (May 2017: 2 zones, €1.65). I could not get a confirmation, but I heard rumours that the crossing would be free for pilgrims.
This might be to prevent pilgrims walking along the tracks over the railway bridge. This unofficial "crossing" is mentioned in some guidebooks. However I heard from locals that sometimes Police would be called to "welcome" the bridge crosser and to cash in a fine.