The bus driver was among those killed, Argentine Transportation Secretary J.P. Schiavi said.
The force of the arriving train reduced the bus to a fraction of its width as it became wedged against the station platform. The front of the train then slammed into another train that was preparing to leave the Flores station in the opposite direction on the Sarmiento line, which connects the suburb of Moreno to the Once station downtown.
Schiavi said children also were among those injured in the accident, which happened at 6:15 a.m., just when many parents use public transportation to take their children to school.
Schiavi said the barriers appeared to be functioning normally, but reporters at the scene challenged that, saying some witnesses had reported that one of the barriers had descended only part-way down, leaving room for the bus to try to drive across the tracks despite the warning bells. Schiavi said investigators were studying videotape of the accident to determine exactly what happened.
Helicopters helped carry the injured to at least seven hospitals around the Argentine capital.
Buenos Aires' passenger rail system moves at street level through most neighborhoods of the capital and surrounding provinces, trying the patience of drivers who often can be seen ignoring the lights, bells and barriers that signal an approaching train. The potential for collisions increases at rush-hour, particularly next to stations, with trains passing and barriers dropping every few minutes.