I can see exactly one use for “light rail”: connecting an outlying airport with a city center and population centers or secondary-business cores on the far side of the city. The El’s blue line in Chicago, the Seattle light rail system (one line just as I described), SEPTA’s train to Philadelphia International,. . . all get a great deal of ridership just as airport links, and I suspect, between that and their ridership as public transit for locals would all be profitable as stand-alone rail lines (unlike most public transit).
Chicago has a good system. It takes you to where you want to go (including both airports and most stadiums). It doesn’t drop you off in the middle of nowhere.