Why one narrow guage versus another?

I can understand the economics of selecting 24 inches as a gauge to build a railroad versus 3 feet. Traffic and cost of construction, the economics of the project dictate the choice.

I can also understand a nation that uses the metric system selecting meter gauge over 3 feet and vice versa.

But why were some lines built to gauges of 3'6" versus meter gauge or 3 ft?

Why 30 inches instead of two feet or three feet?

Where there actual engineering reasons for these decisions?

Could it be political or military?

Or did it come down to the whim of the engineer who designed the railroad?