Why are there double train tracks on railroad bridges?

Why are there double train tracks on railroad bridges?

Hey there! 

Recently, a question has been bugging me, I'm sure a lot of you have noticed this too, but the train tracks on a bridge are always doubled up, like this:

Note the rails are doubled instead of single.

So, my natural reaction was writing a blog post about it.. And here we are, talking about trains

After some research, two possible answers came up, both are extremely interesting and.. I think both are true!

The uses of guard rails (check rails):

1. Double tracks reinforce sleepers (railroad ties) and prevent them from moving in and out of place:

Sleepers/ties are defined as rectangular supports placed under and perpendicular to the rails. sleepers function to transfer the weight of the train to the rockbed below and also to hold the rails in place and at the correct gauge.

The middle stuff are the sleepers/ties

That being said, the main rails hold the sleepers in place and the sleepers give support to the train passing by. But on a bridge, the expansion of the bridge occurs at different rates than the track it self, thus a system is designed so that the railroad tracks are semi-free to move.

That leaves the question of supporting the sleepers/ties. Since ties transfer mass onto the ballast (rocks underneath the railroad), they're essential.

These ties on bridges are supported by a second layer of rails, thus the double rail we see on bridges.

That was a very satisfying explanation, but I remembered seeing double tracks on sharp turns too..

2. Double railroad tracks hold the train in place during turns and reduces centrifugal force of the turn

Centrifugal force represents the effects of inertia associated with turning and which are experienced as an outward force away from the center of rotation. thus the likelihood for our train to flip onto its side.. 

Why does this ALWAYS happen to CN rail?

The double rails help minimize this tendency to swing outwards in a turn by contacting both sides of the inner wheel if the wheel lifts off the track because of centrifugal force. 

Its essentially the lifeline of the train in the case it makes the turn way too fast.

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In summary, guard rails secure the railroad ties in place on a bridge and also prevent derailment by adding support for the inner wheels of the train.

Note how the wheels almost clip on to the outside and inside of the tracks.