How far in advance to reserve a seat on train Vienna to Budapest?

They are still selling discounted tickets for more than 3 days ahead and are still offering reservations with full priced tickets for tomorrow so you should be OK. Mornings seem to be busiest, they are pretty much hourly.

In December I had no trouble buying a ticket on RegioJet from Budapest to Vienna just a few days in advance. Many additional trains offered by OBB and MAV. So for September I wouldn't be concerned.

My last trip Budapest to Vienna and my last trip Vienna to Budapest (last week) were sold out. It was standing room only and, in my carriage, there were about 8 standing in both directions. Someone advised me on the forum a few days ago that train travel was the civilized way to travel. These two trips were not that civilized. Of course, I traveled in July and September will be much better. My trip to Vienna and back that I did last September, things were much better. Several empty seats.

You can generally get the discount tickets up to the night prior, but only if they haven’t all been sold as there is a quantity limit. I would buy it 2 or 3 weeks prior. Check prices with OBB https://shop.oebbtickets.at/en/ticket?cref=oebb-allgemein and with MAV https://jegy.mav.hu/ . Generally, but not always, MAV has better prices as it either sells fewer tickets and as a result has more discount tickets available or starts off with more discount tickets. Not sure which. Doesn’t matter otherwise who you buy from. Same train, same seats. Once you buy the tickets, from whomever, download the app to save and present the tickets to the conductor. Just easier.

There is also RegioJet the private rail company. It can cost yet less, and they offer some seating perks that are sort of airline like in character. The problem is that they go to Kelenföld Station https://youtu.be/VeFb1tQoxg8?si=lWvv4T7WFeSqiEeX in Budapest or you can stay on the train 15 more minutes to Deli Station in Budapest. Both stations are fine. From both stations there is a metro line into town. But these are post-war bus-stop looking stations with very little protection from the weather and absolutely no aesthetic value. If this is your first trip to Budapest it’s a lot more impressive to pull into the classic old Keleti Station https://youtube.com/shorts/TKmyfRGYF10?si=UsuLR1vL37q_1wrQ . But in all fairness all three work. Up to you.

One final heads-up is that of the last four trains I have been on to Vienna or back each and everyone has been late. Significantly late, like 30+ minutes. So don’t plan anything too close to arrival time. I blame OBB.

And do not forget this: 7 thru 18 Jewish Cultural Festival This is good, very good https://www.zsidokulturalisfesztival.hu/programok-2025

Sorry, one more thought. The SideTrip tours looks pretty nice. All the stops are interesting. If you leave at 9am and arrive at 7pm and the total drive time is 4.5 hours, then you have 5.5 hours for sightseeing divided by Bratislava, Esztergom and Szentendre which is a bit tight. But vs never seeing them at all, I think its pretty okay.

Or you could do a early train to Budapest and with the extra time that buys you in Budapest you could run up to Szentendre one afternoon and spend a bit more quality time there; then take the boat back into Budapest. Its a easy sort of fun trip. Happy to show you how.

Trains don't sell out. It would be very unusual for you to be unable to buy a seat reservation on this route. Very unusual especially in late September. You can buy a seat reservation even the day before the trip. It costs just a few Euro. But you have plenty of time - this is not something to worry about.

Before last week I would have agreed with Emily; even argued on her behalf. In general terms Emily is among the best on the forum.

On OeBB's website trains are flagged if there is a high demand, meaning get your seat reservation better earlier.

My last trip Budapest to Vienna and my last trip Vienna to Budapest
(last week) were sold out. It was standing room only and, in my
carriage, there were about 8 standing in both directions.

The fact that there were standees on the train does confirm that strains still "do not sell out". They sell tickets to whoever wants one, and yes, sometimes that leads to more people boarding a train then there are seats.

What you experienced was not a sold out train, but a full train. And I am pretty sure those 8 standees were happy that they could travel, and had not been confronted with a "sold out" train, like in other countries.

Seat reservations of course can in theory all sell out. But it is also rare for that to happen. So you can have the situation where a smart person whips out his phone half an hour before departure, gets a reservations, and then boards his train, finds his seat and sits down, where people that bought tickets without reservation indeed have to stand.