We would like to travel from Budapest to Salzburg via fast train. Can anyone tell me what train and where to book? Thank you.
On the Austrian site I do not see how to book from Budapest.
Maybe it will take a little practice. It took me about 45 seconds to get to the payment page for a Sparschiene (discount advance nonrefundable) ticket from Budapest to Salzburg on May 6 with an 11:40 am departure from Budapest Keleti station. 51.90 EUR with a seat reservation.
Click on the "Book ticket now" block on the bottom of the page and you will be taken to the ticket shop site.
Top of the ticket shop page see the block that says "Now" adjust that to the date and time you want and click on the green "OK" button. Then in the "From" box type in BUDAPEST (all caps means any station) on the "To" box, you'll see Salzburg. Select that. Then you get a "Ticket" block, click on it and available trains will populate. select the one you want and type of ticket (1st class, Sparschiene, semi flexible, flexible) with prices. Select what you want and put it in the "shopping cart".
I see from your other post, you are going to Budapest in September. It is way to early to buy train tickets now. There is a general European schedule adjustment due 2nd week of June.
As your journey originates in Budapest, you should purchase your ticket from MAV .
Here are the instructions - https://www.seat61.com/international-trains/trains-from-Budapest.htm#Budapest-Salzburg
That said, you are too early now to buy tickets.