extra Rails?

[jjjim <jjjim@bigpond.com>]

Hi peeps,   I have noticed this before but took extra attention of it on
the way home today:

ON what apears to ONLY be on sections of Track that are going over some
sort of bridge (say a creek  or over a road)  Are a second pair of Rails
Right in the center of the track,  right next to eachother,  , say about
20cm apart  ,  I was wondering what there purpose was?.   I had an idea
that maybe it was a saftey thing in the case a train was to derail on
the bridge, helping to  prevent it from running right off the track..
thus off the bridge and crashing into the creek / Road below!  which
would no doubt cause  massive carnage  ,  would this be the reason, or
am i way off ;)