I've ridden on light rail in every country I've ever been in, except Mexico and India.
I've ridden on exactly two systems in every country but the US, Mexico and India:
Japan: Sapporo Subway, Osaka Subway.
Australia: Melbourne Trams and Sydney Monorail
UK: Glasgow "low level" trains, London Underground ("Tube")
Canada: Toronto subway, Montreal Metro
In the US I've ridden quite a few more:
Chicago CTA, Seattle Monorail*, Atlanta Marta, San Diego Trolley, Philadelphia Subway, Disney World Monorail*, NCTD Sprinter (San Diego County), Disneyland Monorail*
* Indicates systems that are more tourist attractions or theme park rides than practical transportation, although all 3 do work to get someone from place to place, and I've used all 3 as such.
For what it is worth, I've also ridden on some "heavy" rails:
Cumbres & Toltec Scenic Railroad (twice), Durango and Silverton Scenic Railroad, Chicago Metra, Amtrak (Southwest Chief, Pacific Surfliner), NCTD Coaster (San Diego County)
If my current travel crystal ball holds, I'll probably not get any more light rail systems for the next few years: next year I'll just be repeating systems in Australia, I don't expect to use light rail in Reno, Chicago would be a repeat, as would either Glasgow or The Docklands, and I don't think San Antonio has light rail.
Of course, if Japan wins Worldcon for 2017 and we can somehow afford to go, that would likely add some Japan systems to my log. An unexpected business trip could also add systems depending on where I were to end up. But I'm not expecting any at least not in my current position.