The Metropolitan Atlanta Regional Transit Authority (MARTA) operates heavy rail trains in the Atlanta metro area. All trains travel underground at different points along the rail line, so technically MARTA is a subway when this occurs. Trains also travel above ground, especially as you get further away from downtown Atlanta.
Because Atlanta is very sprawled, MARTA functions as a hybrid transportation system. Near the city’s business districts, stations are close together and are similar to the subway system in New York City. Stations that are near the end of the four rail lines are further apart and trains function more as a commuter rail service.