Train tickets from Budapest to Vienna on the Austrian Railway site

You can travel on RJ from Budapest to Austria without a reservation. Which website were you using, MAV or OBB? The OBB site works but has a slightly illogical step and what may be a translation error.

Here is a link from the Man in Seat 61 website with instructions. Rule of thumb is that you want to purchase your tickets from the train company of the country where your journey starts. In your case that is MAV, not OeBB.

https://www.seat61.com/international-trains/trains-from-Budapest.htm#Budapest-Austria

You do not need reservations on RailJet trains. You can take any seat that is not reserved. If you want to be sure, however, that you are all sitting together, then you should reserve seats. You can buy at seat reservation in Budapest at the train station a day or so before you travel. There is plenty of room for luggage.

... we haven't been able to get past just buying unreserved tickets ...

What was the problem? Although the OeBB website is known for being far from user-friendly, I bought tickets from Vienna to Budapest several times without problems. One issue - often discussed in this forum - is the requirement of having the 3D-Secure feature activated on your credit card for online payments.

Buying seat reservations can be done together with the tickets, or later separately.

I always buy seat reservations because I don't like to walk through the train carrying my luggage searching for vacant seats, let alone that I prefer to choose seats at a table for my laptop.

I just bought the tickets at the train station. No big deal.
Don't be surprised at how slow and old the train is. They're not the clesn, fast trains you'll get in some.other places.

wmt1,
We didn't even get to the payment part !
But if it's possible to buy unreserved tickets now and then go to the Station a few days prior to our journey and pay for reservations, that's what we'll do.

David obviously hasn't travelled for Budapest to Vienna in recent years.....

http://www.oebb.at/en/leistungen-und-services/railjet

Yes, the Railjet service between Budapest and Vienna is the epitome of clean and fast!

@Golfing65: I assume that your statement about "...haven't been able to get past just buying unreserved tickets" means that you didn't see that option on the OeBB website. I just bought Budapest to Vienna rail tickets on the OeBB website last week and didn't see that option either -- but when revisiting the website later discovered that I didn't scroll down far enough after selecting the itinerary and class to see the option to purchase seat reservations (it was at the bottom of the page). Since I had purchased my tickets without seat reservations and desired them, I emailed OeBB customer service and, after 10 days (!!), received the following instructions as to how to purchase seat reservations only if you already have a ticket: "In our online ticket shop you can book separate seat reservations without a ticket as follows: please enter your travel details and number of passengers and select the connection. On the next page under "Our services" please choose "more results" and "Ticket already exists". Then you can select your reservation requests under "Reservation details". The cost to reserve seats for our one-way trip was 3 Euros per person. Hope this answers your question.

Thank You everyone !
We did manage to buy our tickets, albeit without reservations !!
The next hurdle will be to get reserved seats, but will all these instructions I'm sure we can do it.

The connection is now between Budapest Keleti and Wien Hbf, used to be with Wien Westbahnhof before Wien Hbf became operational. No, reservations are not mandatory for the 0840 departure. If you can want to ensure yourself of a seat, get it at the train station.

If you can want to ensure yourself of a seat, get it at the train station.

There is a deadline for seat reservations because the reservation information has to be loaded into the displays above each seat prior the train is provisioned at the platform for departure. I do not know how this is handled in Budapest, but I would buy the seat reservations one day before travelling at the latest.

True on getting the train reservation. for Wien.