Wien / Vienna to Graz by train

OBB is the national rail operator and its booking service offers journeys by express trains within Austria, as well as international journeys which don't involve making a connection outside of Austria.

It pays off to book online, as the discounted 'Sparscheine' tickets are only ever made available on the website, and when making international journeys to and from Austria, these Sparscheine tickets tend to be cheaper.
OBB can offer particularly good value for money when travelling with children on international rail journeys, but something to watch out for when travelling between Austria and Germany on daytime trains, is that reservations are an optional extra; but if you book 1st class tickets with the Germany railways website DB, the reservations are complimentary.

OBB often takes a unique approach to its booking path, but a key thing to keep in mind is that the first price you will see for any journey is that of a Second Class seat.
The costs of upgrading to first class, a reservation, or a sleeping cabin etc, is then added to this price.

Happy Rail is a Netherlands rail ticket agency which sells tickets for both national and international journeys within a range of countries including Belgium France, Germany, Sweden, Switzerland.

HappyRail doesn't charge booking fees in the conventional sense, but if you're not Dutch you can expect to pay additional transaction fees, which will be added to the total cost.
That's because the most common form of online payment used in The Netherlands is 'iDEAL' and HappyRail doesn't charge a transaction fee for 'iDEAL' payments, but only Dutch residents can sign up to 'iDEAL'.
Other forms of payment include Visa debit cards will incur a fee.