The distance between Vienna and Budapest is only 215 km, which means a slightly two and a half hour traveling. Direct trains run between the two capitals every two hours to both directions! Trains leave from Hauptbahnhof Train Station in Vienna and arrive at Keleti Train Station. At Keleti station there is the M2 and M4 subway intersection, so you can easily get to almost every part of the city from here.
13 trains run from Vienna to Budapest a day, from 5:40 a.m. until 20:40 p.m on weekdays, holidays and at the weekends as well. On the line of ÖBB (the Austrian Railway Company) you can choose between EuroCity and RailJet.

When traveling from Vienna to Budapest, I usually travel by RailJet trains, I like that they are really clean (as shown in the attached photos). You can choose from three classes on these trains, the business, first and second class. My favourite is the business class, where you can travel in really comfortable armchairs which makes your travel very convenient!
Good to know that you can travel on these trains without booking a seat, and sit anywhere, but if someone has a reservation to where you are sitting, you have to give it up to them.
If you arrive in Budapest too early and cannot occupy your accomodation, it is worth using a baggage room in the city or at the railway station.

Business, first, or second class?
The business class is with the highest level of comfort, which is actually the first class with extra services. You can buy a ticket for this class for a surcharge of €15 and please note that you cannot take any pets with you here.
In first class, if two of you are traveling, it is worth reserving tickets next to or opposite each other which come with a table.
In the second class: there are seats with tables or without tables. I think it’s better to choose seats that come with a table!
Put your baggages in the storage above your head if the bags are not too big! Separate storage space has been developed for larger bags, but be careful when using that, don’t let greater value in the bags, and if possible, check them sometimes!

Tickets from Vienna to Budapest by train
You can buy your tickets online or at the railway station. When purchasing online on the following websites, take note of the prices as they can start from €13 when booking in advance, but if you buy tickets in the last minute, they can cost even €40!
When you are planning to go from Vienna to Budapest by train, it’s worth checking out both of the websites because you can find different sales and discounts on them!
If you buy ticket on the Austrian website, you will get them in pdf file that you can print anywhere, and so they will be accepted on the train.
You can get SparDay tickets for direct trains from €13 for the second, and from €29 for the first class.
Seat reservation: €3, for Kálmán Imre Euronight car: €5 and for RailJet business class: €15.
As I know it, a special euregio ticket can be purchased for children under the age of 14, but that my be available only for citizens of the European Union.
Prices of standard tickets: €47 for second class, and €75 for first class.
If possible, don’t exchange money at the train station because it’s pretty expensive! Tickets for public transport, or if traveling by taxi, can also be paid with a credit card.
In short: you can go from Vienna to Budapest in the most quickly and comfortable way by train, choose one of the trains running every two hours and purchase your tickets as soon as possible!