Why would you want to do such a thing?
Ian
the uk 5 inch gauge pages here might shed some light http://www.rideonrailways.co.uk/page24.html
rail is available in aluminium and sleepers in plastic - this would be lighter than a steel /timber track which is the usual .
I would suggest short lengths an add spacers to each end from aluminium angle to ensure a rigid structure - normal fishplates to join lengths with alln head screws to facilitate quick power tool assembly .
It comes down to how strong you are - or how strong you want to be.......................
http://www.miniaturerailwaysupply.com/index.htm also has a lot of info
That could get a little tiresome.
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