There's a very comprehensive list of current and obsolete gauges worldwide at Railroad Gauge Width
. I've referred to it often over the years, but my current painfully slow internet connection discourages me from doing much internet research these days so I've only just rememberd it and checked it. I'm not sure whether it is completely up to date, as standard gauge is burgeoning in a number of African countries over recent years. As far as I can see it doesn't show South Africa (whose standard gauge Gautrain is now several years old), Kenya or Ethiopia as having standard gauge.
China and the OIC have announced a Port Sudan to Dakar rail link, over 10000 km across the entire width of the continent, which will almost certainly be standard gauge, as part of China's new Silk Road trading strategy (link