Just book at any station in Romania. I've never had any problems with that.

Thanks! Did you have to pay extra to reserve the seats?

Yes, but not a lot. You can find the prices on this page.

Sorry for asking so many questions - could you reserve the seats on the day or it had to be several days in advance? We are arriving in Bucharest and planning to travel to Brasov right away, so there is no opportunity to book in advance unless we can do it online somehow.

MOST often on the day for next train. It is an age-old tradition that all longer-distance trains in RO from CFR have mandatory seat-reserv-it just comes for the next train if nothing is specified. In fact untill not even too long ago it was not even computerised and done by complicated manuals.

NOTe that there are besides CFR=-the old communist state railway now also several other private companies that run on this line and these do NOt take any pass. IF you have a pass with traveldays, most likely even a normal ticket will cost a lot less as1 passday. There are also trains from 1 of the 2 airports-also both from CFR and another.

BUT as the govmt gave loyalty to its voters there are now big discounts for studenst and some others-these overcrowd trains on FRI -late FROm BUC (or less so from other uni-cities) and back to there SUN afternoons. For those its best to try advance. Brush up your romanian. Also for overnight best to do some days advance.

It was a few years ago but we've often bought reservations on the day. Since 30 May is a normal Tuesday, I wouldn't expect trains to get fully booked.

I've checked the information on the Eurail website (click on "More information” when you select a train in the reservation module). The Eurail website should also be able to book a train in Romania (as always with a 2€ booking fee), but only up to 30 days in advance. Romanian reservations are emailed so that should normally work fine, but there seems to be some kind of problem today because no Romanian reservations show up at all, so just try again later.

Interestingly, you can check the number of available places in a Romanian train on the CFR website. Even for trains Bucharest - Brașov today or tomorrow there are plenty of seats left.

Thank you guys, this has been very helpful! Oddly, the CFR website appears to be down for me in Australia. 

Yes, I forgot to mention: I could only reach it by using a VPN set to Romania. I assume they're suffering a hacker attack, so this is probably a temporary measure.