Police said 19 people died. Roberto Capiel, a public health official, told Telam that 47 people with injuries were taken to a local hospital for treatment. He said nine of the victims remained in critical condition.
Television footage showed wreckage of the overturned bus with its front windshield shattered and much of its roof sheered off. Crumpled bus seats, children's clothing, a baby bottle and beach chairs were left scattered across the railway line after the wreck, which took place at about 2 a.m. local time.
Bus passenger Cecilia Demarco told Noticias Argentinas that the bus "tried to beat the train" to the crossing, despite repeated warnings from the train as it sounded a horn. The account could not immediately be confirmed.
A Todo Noticias broadcast showed images of crushed motorcycle under the debris and said authorities reported that a woman waiting on a motorcycle at the crossing was among the dead.
A survivor on the train who identified himself only as Alejandro told the network he was preparing to go to sleep when the collision occurred.
"I heard this tremendous crash and everything shook" the witness said. "There were many screams, and all of this in the dark."
Police said the bus, which was bound from the beach resort of Mar de Ajo to the capital, was carrying 61 passengers and two drivers.
Authorities said there were at least 30 severe injuries reported on the bus, and at least 15 train passengers and the two train drivers received minor injuries.
Buenos Aires provincial Gov. Daniel Scioli visited the crash site and promised an exhaustive investigation.
"It's just devastating, so tremendously sad. And to think there were families going on vacations is a disgrace," Scioli said.
Authorities said the train's drivers told investigators that the crossing barriers were down when the bus unexpectedly tried to cross the tracks.