If you are flexible with your travel dates, always consider that you can buy extremely cheap train tickets if you avoid travelling in the weekends and if you choose to travel during the low season.
If you don't feel like spending many hours in a train, you can consider travelling by plane. Especially if you have the possibility to book in advance, flying to Buenos Aires can be definitely cheap.
In which stations can you take the train to Buenos Aires and where will you arrive?
The trains to Buenos Aires can leave from Bahia Blanca (Buenos Aires - Argentina) and arrive at Buenos Aires (Retiro Cap.fed - Argentina).
What time do the trains to Buenos Aires leave and how long is the journey?
The first train to Buenos Aires leaves at 06:12, whereas the last one at 23:12. On average, the journey lasts 9 hours, but with the fastest connections you will be in Buenos Aires in 8 hours.