>>1666263In the past most trams went either with standard gauge or metre gauge, usually related to whether streets were wide and curves ample so you could use larger vehicles, or if streets were small and turns narrow you'd get smaller trams that could do narrower turns. For example in the US almost all tram systems used standard gauge, since they didn't have small old towns, so they could use xboxhueg vehicles. In Europe however many cities used narrow gauge so the trams could navigate the small old towns. In Switzerland to this day all tram systems use metre gauge and will never be changed to standard gauge. Metre gauge is a reasonably common gauge for trams so I don't think it's much of a cost difference, unlike other unusual gauges like 900mm.
Modern 2nd gen trams are almost always built in standard gauge as they are usually meant as high-capacity systems running large vehicles on big main streets or ROWs, and like other anons said, you can get off the shelf vehicles for them. Other than that I don't think theres really any advantage of std gauge over metre, since trams won't usually travel so fast that this would make a difference.
Another advantage is if you want to make a tram-train line using conventional rail tracks, in which case you'd use the gauge of those lines, usually standard gauge, but in Spain they built a tram-train in iberian broad gauge.
