AussieAndy Most trains have a second part of the train who stays in Feldkirch or continue to Bregenz, try to book only to Feldkirch, you will have a seat until this station.
If you are prepared, you can change in Feldkirch and take the Swiss part of the train or you can continue with other regionals trains, Bregenz - St. Margrethen - Zürich is an alternative.
But your example is for today, not for the 20th October. My advice is valid only for last minute reservations when the train to Zurich is sold out.