Mar del Plata
While Argentina's only true subway is in Buenos Aires, there are active plans for metros in Mendoza, Cordoba and Santa Fe. Tren urbanos in Tacmon and Salta are also running and function as urban commuter lines. There is also talk of a 22 km light rail line in Salta.
Argentinian Railway Sites
Argentinian rail blog - great fodder!
Salta Tren Urbano
Tren de las Sierras - also not a subway but uses metro-like cars.