Comment by red_admiral

Heavy and Light Rail in the UK is also a safety distinction: Light Rail is limited in speed, drives on sight (exceptions apply) and a few other restrictions (https://www.orr.gov.uk/about/who-we-work-with/railway-networ...).

Trams are one example of Light Rail, but so is the Docklands Light Railway in London (an exception to "on sight" - it's automatic), as opposed the the Tube (underground) which is Heavy. But the Welsh "metro" project's "tram-trains" (Stadler Citylink) are also Light, even though their tracks into the valleys are very much not urban in the usual sense - the full valleys journeys are over one hour with the current trains and go through mostly rural areas with small towns.