? ?No worries, there's no need to book anything. The procedure is pretty straightforward, just go to the station and buy your ticket some 15 minutes before the departure.
? ?Trains operating between Serbia and Romania are usually almost completely empty, so you're very likely to have the entire compartment all to yourself, which is almost like a sleeper
? ? You may need to stress out that you want the 2nd class, otherwise they'll do their best to sell you the sleeper, which is more expensive and you don't really need it.