Recommended because the train can get busy (still optional but you might have to stand or swap seats along the way). Around Christmas & New Year expect trains to be busy.

I’d get reservations on Prague - Vienna for sure : they’re free with a 1st class pass through https://shop.oebbtickets.at/en/ticket ! Add Eurail as a discount, select one-way tickets and then you have access to a seat map.

After a quick look :

• 08:44 train : 28 seats left
• 10:44 : 6 seats left, not contiguous
• 12:44 : 22 seats left

The 1st class area is quite small on these trains. You might prefer to sit in 2nd class : you’ll get seats next to each other the train will be quite full nonetheless.

You could also travel in “Business class” : very very spacious, 15€ surcharge available on OBB. You’d sit next to each other. Pictures here : https://www.seat61.com/trains-and-routes/railjet.htm#What_are_railjet_trains_like (would be open plan version)

Meanwhile Swiss trains rarely offer reservations. And even when they do nobody makes them, the system doesn’t work around reservations here (unless you’re a big school class). You hop on and sit wherever you want. Expect 1st class to be pretty empty, 2nd class is expensive enough for most people :)

Hi, thank ypu for your assistance. We choae the 12.44 time. I have log in  OBB, and added the Eurail for discount. Selected the seat reservation only and choose the seat number.  There is a charge of €9.00. This is correct, right? 

1st class will be seated on Couch 27?

For the free reservations you must select one-way tickets (and not seat reservation only). Counter-intuitive but it's the only way that works !

You won't have to enter payment details whatsoever.

EDIT : 1st class is indeed coach 27.

"Meanwhile Swiss trains rarely offer reservations. And even when they do nobody makes them, the system doesn’t work around reservations here (unless you’re a big school class). You hop on and sit wherever you want. Expect 1st class to be pretty empty, 2nd class is expensive enough for most people :)" - as  per the previous message, we did the hop on and sit wherever we want. The only downside if the seats has been reserved by other passenger then we have to change seats again. Is it really possible we can sit on 1st class couch? 

Which were you on ? I'd be really really surprised if someone made a reservation (except tourists). I've lived here all my life and don't know anyone who ever made one.

The system just doesn't work around reservations. You hop on and find a seat, if the carriage is busy go to the next one. 99.9% of the time it works, even more in 1st class.

We are now seated at Couch 1. Yes, it happened to us now, we were moved twice since everyone who hop on has seats reservation. I guess becauae this journey is going to Milano Centrale. (Trenetalia)

We will be back to Basel after 2 days and same the seats reservation is still optional. Which carriage/couch do you advise we should hop on to avoid this kind of situation. Thanks. 

Ah yes this is different because reservations are included on tickets to Italy (mandatory after the border).

Often on trains to Italy one unit goes to Milan and the other stops at the border (Domodossola/Chiasso), that one is quieter.

Check SBB app : it should tell you how busy each carriage is. The rule of thumb is to go to the front/rear end of the train -> people are lazy and board in the middle near the stairs.