Swiss Railways (SBB/CFF/FFS) operates a train from St. Gallen to Rhine Falls hourly. Tickets cost $24 - $45 and the journey takes 36 min. Thurbo also services this route hourly.
How to get from St. Gallen to Rhine Falls
From St. Gallen train to Winterthur, line 33 train to Schloss Laufen am Rheinfall, then walk to Rhine Falls. 1 h 35 min SFr 48. From St. Gallen train to Schaffhausen, Bahnhof, line 1 bus to Neuhausen, Zentrum, then walk to Rhine Falls. 2 h 12 min SFr 17. From St. Gallen drive to Rhine Falls. 53 min SFr 12.