Trains... Choo Choo, not the dirty kind.

[Carruthers]

I think that this might be the answer, but will stand corrected.

Why do they have the two extra railway lines (in between the main lines) on bridges?

Do you mean the guard rails placed parallel to regular running rail along areas of the bridge?
These have the effect of keeping the wheels of rolling stock in alignment in case of derailment(making sure the derailed train will not turn over).
It also helps to minimize damage to the bridge and allow easier post-accident cleanup.
from Wikipedia: Guard rails (railroad)

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Perhaps a better example...



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