Kevin,
There was a consensus aroound 3 feet that believed that larger than 3 feet did not deliver enough economy from downsizing from standard gage to justify the interchangeability hardship of being non-standard. In the other direction, the consensus was that smaller than 3 feet sacrificed too much hauling capacity. If downsized further without sacrificing capacity, it sacrificed stability.
But then there is also the question of why 36" rather than 35"? With this issue, it simply seems as though a consensus loves a round number.