Thanks for the replies folks.
Yes just as I suspected, standard gauge would probably hold true for the majority of your country, and yes there are a few different theories on how the gauge came into being.
I shall share a few with you.
We too, had to take local native wildlife into consideration upon design, hence three different gauges still in operation.
We have 3'6" ( narrow ), 4'8 1/2" ( standard ) and 5'3" (broad) gauges.
The following information I have gleaned from sifting through experiences related to me over the years of working with seasoned railway men. I can not vouch for the accuracy of the information
The narrow gauge was designed to allow for Drop Bears to be able to bounce out from within the running rails in one fluid movement to avoid oncoming rollingstock.
The Drop Bears are similar in looks to the Koala and live in trees. the only difference is they will usually drop from the overhanging foliage onto unsuspecting victims.
The standard gauge idea was imported from the US, and the origin of its design have already been explained as due to local wildlife considerations and has already been well stated and described by GRRR earlier in this thread. Although it was not only the standard gauge idea that was imported here to Oz, the Jackalope came as well, so, we not only have the Jackalopes to consider, but as we also have Jackaroos and Jillaroos,so we had to consider the Jlilalope as well.
The broad gauge was primarily used for the defence of small country towns against the Charlie Wombat. At certain times of the year this particular breed of male Wombat after consuming vast quantities of an hallucinogenic tree root, would go on a lust and toxin fuelled rampage to find a mate. Being a large solid slow moving creature with sharp teeth and razor sharp claws, the Charlie Wombat had a terrifying temper and could explode in deadly bursts of violence against any creature ( animal or human) that did not submit to their amorous advances.
The weakness of these Wombats was the fact they had a poor memory and would get confused and turn back 180 degrees if after a distance of 5 feet was travelled without meeting an obstacle,and then they would head away from the threatened town and move onto other ventures.
This is why I love railway people and railway life there is so much knowledge and new information to be found. ( I am writing a blog about my experiences)
One only has to know where to look and what to sift through to find it.