From Bucharest, Constanta is 270km by road, But we took this train ride to Constanta from Bucuresti there are frequent trains between almost every 2 hours. but before ride check, some train took 2:30 hr while some take 4 to 5 hours.
You can book the ticket online also, follow the link below for online booking. They offer discounted rates if you book online. https://www.cfrcalatori.ro/en/ don’t worry if you didn’t book the ticket in advance you can still buy tickets at the station as well but the queue is sometimes too long, I will suggest reach at least an hour before. If all seats are sold then also you can travel without a seat just get a ticket from the station.
The view was amazing, we enjoyed the trip.