Rail Ninja and RegioJet come up from time to time; most recently in reference to trains from Vienna to Budapest.
Rail Ninja is a train ticket re-seller. RegioJet is a private train company. OBB and MAV are the Austrian and the Hungarian national railway companies.
Just some comparisons. Things do vary a bit depending on the train you select, but in broad terms. Buying anything from Rail Ninja will be the most expensive. Sometimes just a few euro like the example below, sometimes it can be double the cost. On this route the second most expenive is purchasing directly from OBB. The cheapest using a national train company is the Hungarian national company MAV. But the cheapest overall is the private company RegioJet. RegioJet does not always go to the main station in the cities it serves, so check on that as well. In this case, from the train station in Budapest served by RegioJet you still hae a 30 minute trip into town.
October 9; Budapest to Vienna 140 HORTOBÁGY EuroCity / 08:40 / 2:40 travel time / 2nd class
Rail Ninja 60 euro, includes seat reservation.
MAV 29 euro (11310 HUF), add 3 euro for seat reservation.
OBB 52.52 euro, add 3 euro for seat reservation.
RegioJet RJ1032 / 07:58 / 2:37 travel time / 2nd class (closest time to the one used for MAV / OBB
11.90 euro, add 2 euro for seat reservation. Tran goes to the Kelenföld station. Allow 30 more minutes to reach town center.
October 9; Vienna to Budapest 143 TISZA - SZAMOS EuroCity / 10:42 / 2:37 travel time / 2nd class
Rail Ninja 51 euro, includes seat reservation.
MAV 23 euro (8970 HUF), add 3 euro for seat reservation
OBB 52.52 euro, add 3 euro for seat reservation.
RegioJet RJ1032 / 10:27 / 2:31 travel time / 2nd class (closest time to the one used for MAV / OBB
13.90 euro, add 2 euro for seat reservation. Tran departs the Kelenföld station. Allow 30 more minutes to reach the station from the town center.
It always pays to shop around. The other recommendation that comes up often is to use a RailJet train because they will be faster. Not necessarily, or not necessarily that much faster. Just choose the schedule that works best for you at the price that works best for you that has a train that is within a few minutes of the fastest. 4 minutes longer might be worth a 20 euro savings.