Complex sentence: Fifty-one people were killed and more than 700 were injured; the dead and seriously injured were in the first two carriages, which was packed with people who had moved to the front of the train to be near the station exit on arrival.
Split into simpler sentences: 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.