The 2012 Buenos Aires rail disaster, also known as the Once Tragedy, occurred on 22 February 2012, when a train crashed at Once Station (Spanish: Estación Once de Septiembre; IPA: [ˈonse]) in the Balvanera neighbourhood of Buenos Aires, Argentina.

There were about 1,000 passengers on board when the crowded eight-carriage train, whose working brakes were not activated, hit the buffers at the end of the line, crushing the motor carriage and the following two carriages, after approaching the station at a speed of 26 km/h (16 mph). Fifty-one people were killed and more than 700 were injured; the dead and seriously injured were in the first two carriages, which were packed with people who had moved to the front of the train to be near the station exit on arrival.

The Sarmiento Line, on which the incident occurred, was operated by Trenes de Buenos Aires (TBA), owned by the Cirigliano brothers. It was the second fatal accident on the line within six months, following the 2011 Flores rail crash, and the third-deadliest train accident in Argentina's history, after the Benavídez rail disaster in 1970 (which left 142 dead and 368 injured) and the Sa Pereira rail disaster in 1978 (which left 55 dead).