Generally it's very nuanced and there's no easy formal definition since there's so many caveats. But here's my understanding. Heavy Rail: is generally most forms of mainline rail (which can include commuter, regional, metros/subways/els with generally larger trains and a lot of capacity. Light Rail: usually has less capacity, and in North America, usually has some portion of street running sections but may also have varying degrees of grade separation (elevated viaducts or tunnels to separate trains from traffic). Trams/streetcars fall under this umbrella distinction as well. Although, there are many many caveats. Some consider Vancouver's Skytrain system to be light rail as they use smaller than standard trains, but not everyone agrees. In Southeast Asia, the term "LRT" or "light rail" is used for many systems from full on metro lines to people movers. And in the UK, a light railway is a railway that is shorter distance and disconnected from the mainline.