"Rail replacement service" usually means a bus instead of a train for all or part of the route. Your ticket is valid on the bus, just as it would be on the train. Look up the route in the timetable for details. If your travel date is before May 12 and you were going via Linz, you'll probably find this:
Due to construction work, this train cannot run between Pregarten Bahnhof and Linz/Donau Hbf. As an alternative means of transport, we have organised a rail replacement bus service for you. Your journey will take up to 10 minutes longer. We apologise for any inconvenience this may cause.