In reply to woolsack:
Some trivia points:
The railway gauge is the indeed the same as the width of a Roman chariot, which was based on the width of the hindquarters of two standard Roman horses.
In the US, it is quite difficult to buy some metric sizes, because practically everything is made in Imperial and Imperial rules. Problem is that everything imported from the Far East or Germany (which covers an large proportion of technological gadgets) is in metric. So anything that is made outside America that has American accessories added ends up having both metric and imperial screws. This is particularly annoying with Allen keys, because one can not distinguish the sizes by eye.
Worse still, the American Imperial units are not necessarily the same as British Imperial: most notably the British and American gallons are different. (The US gallon is 5/6ths of a British Gallon).
The standard conditions under which the American barrel of oil is measured differ from state to state.
The American 2 X 4 is never 2" x 4", in my experience, whether used where visible or not. The "2 X 4"s that I bought to repair part of my garage are actually 1 1/2" X 3 5/8", which seems a much larger difference than can be explained by planing versus non-planing; more like simple short-changing.
Many American houses use almost nothing else but so-called "2 X 4"s in their structure (for cheapness). When thicker beams are required, "2 x 4"s are just nailed together in multiples!
On top of all of this, the US dollar is shrinking!