I would imagine you will be travelling south from La Paz. In this case I would take the train from Oruro, via Tupiza, to Villazon on the Argentine border. Regrettably most Argentine rail passenger services have been discontinued, but there are excellent bus services to Jujuy and Salta from the Argentine side at La Quiaca.
Here is a great web side for rail services on the Bolivian side:
http://www.bolivia.freehosting.net/trains.htmRead our fellow Travelpod bloggers on this area:
http://www.travelpod.com/cgi-bin/keyword2....eyword=villazonhttp://www.travelpod.com/cgi-bin/keyword2...._keyword=quiacaA much longer alternative is from Oruro, via Uyuni and the great salt pans to Antofagasta, Chile.
http://www.travelpod.com/blogs/0/Bolivia/Uyuni.htmlhttp://www.travelpod.com/blogs/0/Chile/Antofagasta.htmlThen over the Andes to San Antonio de los Cobres to connect with the famous "Tren de los Nubes, which runs only on Saturdays from April to November. There is still sporadic rail service from Socompa on the Chile/Argentine border, but I hesitate to give any timetables for this. There will also be good international bus service direct from Antofagasta to Salta.
http://www.enjoy-argentina.org/salta-desti...rain-clouds.phphttp://www.travelpod.com/blogs/0/Argentina/Salta.htmlI have probably buried you in information, but flip through these blogs, as I'm sure you will find lots of great advice from fellow travellers. Remember you can contact most of them if you see something and you need specific information.
Hope this helps.
Buen viaje!