It's all over the place. Probably standard gauge is most common, but there's tons of variation. For instance, the DC Metro is slighly (like a half inch) narrowe

It's all over the place. Probably standard gauge is most common, but there's tons of variation. For instance, the DC Metro is slighly (like a half inch) narrower than standard gauge, but SF BART is MUCH wider, something like 8 inches.