The 12.19 does indeed split at Crianlarich, one half going to Taynuilt and Oban and the other to Fort William and Mallaig. This is usually made clear on the display boards and on board announcements. With the temporary use of the low-level platforms usually employed for local services, and a convoluted route through the Glasgow suburbs, it won't be clear which is the front end of the sprinter units. Challengers usually wander aimlessly through the train and sit wherever they can find a seat until Crianlarich. There is plenty of time there to move to the appropriate coach. And the on board staff usually speak some English.