A standard gauge railway has track with 4 foot 8½ inches (1435mm) between rails. The story of the standard gauge being derived from the width of Roman chariots is likely to be an urban legend.

after http://www.railpage.org.au/comrails/common/html/glossary_s.html

Why such an odd measurement? It makes a lot more sense if you realise that 4' 8½" between the insides of the rails, on the first railway systems, probably equated to a 5' outer gauge, between the outer rims of each wheel, on early tramways. Five feet is definitely a 'round' sounding number.

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