Apparently they'll be in service as a tourist attraction on a limited run service. Starting this Saturday from 23.30 on.
I love those old cars. A real living treasure, a living antique originally from Belgium 99 years old.
If you are into historic train, you can also have a ride in Caballito every weekend and holidays on the historic tranvía: http://www.ba-h.com.ar/buenos_aires/tranvia_historico_caballito_buenos_aires.htm#.Wac1xEqGPgE It's free!