Tangiwai Disaster Cause and Effects.

I have been studying the Tangiwai train disaster.  My question is why did this happen in the first place, what are the cause and effects.  I know that a river was flooded and a bridge couldn't quite handle it.  This all unfolded in 1953 Christmas Eve, 285 people were on board and 151 died.  Most people were heading home for Christmas so they were equipped with presents. At the time this was the 8th worst train disaster in the world.  This was really a tragic ending of the year for Kiwis.
😢😢