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.
Which stations serve the rail line Mendoza - Buenos Aires?
The trains to Buenos Aires will leave from Mendoza (Mendoza - Argentina) and will arrive at Buenos Aires (Retiro Cap.fed - Argentina).
How long does it take from Mendoza to Buenos Aires and when do the trains leave?
On average, it takes 16 hours and 6 minutes to get to Buenos Aires by train, but you will need 14 hours and 15 minutes with the fastest trains. The first one is available at 07:12, whereas the last one leaves Mendoza at 21:12.