G'morning David, 

 DW> Looking down on them from a train passing through is not encouraging.

Heh ... travelling down to London by rail during and after WW2 
gave you a certain insecure feeling...

 DW> A lot of rivers here freeze solid in wintertime. In spring, the ice
 DW> often breaks up suddenly, releasing torrents that drive huge chunks of
 DW> ice onto the river banks. Buildings nearby have to be constructed to
 DW> cope with that, which makes a simple flood seem trivial.

NZ's worst rail/bridge trajedy is undoubted the Tangiwai disaster 
of Christmas Eve, 1953.  The crater lake up on Mount Ruapehu 
(highest volcano in the North Island at 2797m and active) broke 
its banks and released a lahar (a silt-and-water flood) that 
travelled some 20km before tearing out the railbridge at Tangiwai 
just before the night express from Wellington to Auckland got 
there.

Being Xmas Eve and in the middle of a Royal Visit, the express was 
loaded and 151 people out of 285 passengers were killed...

All other rail accidents in NZ pale beside this one,  and They are 
paranoiac about about another lahar;  the lake is brimming again, 
and so instrumented that heaving a stone into it creates a 
precautionary mobilisation of rescue forces.

:-)
  
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