Did anyone travel on the train from Bs As to Posadas? Where is the station (departure?
The journey takes 36 hours...however at a cost of 40 pesos.

I never traveled on that train and personally never would. Most trains are un desastre, but I guess it's you get what you pay for. Here's more info:

http://www.xcolectivo.com.ar/tren/misiones.html