Heavily depends on the train, please give use the route if you want precise advice.

On regional trains reservations are not possible for anyone : simply hop on and find a free seat.

On trains with optional reservations : you can sit on any unreserved seat and if the train is totally full, you might have to stand but it is rare. On busy mainlines it is a good idea to make a reservation, they do not cost much. Avoid eurail.com at all costs.

- free in the UK

- 3€ in multiple countries through tickets.oebb.at (free with a 1st class pass)

- 4.90€ per journey on bahn.com

Actual trip is RJX 660 Salzburg to Innsbruck 17 June 2023 08:56

You can check occupation on tickets.oebb.at : there are 100 seats available in 2nd class so I wouldn't worry.

Optional seat reservations cost 3€ (2nd) or are free (1st).

I have paid for 1st class so are there any 1st class seats available?

There are 22 seats. You can make a reservation free of charge through tickets.oebb.at : add Eurail as a discount and select one-way tickets.

I'd do that for peace of mind.