[ardecila]

The whole concept of light rail when it was devised back in the 70s was to create hybrid regional systems at a lower cost than full-fledged metro/heavy-rail technology could deliver (BART and MARTA demanded huge sums of money and Baltimore/Miami were cut off before they could build a full system). The result was something that was a mix of streetcar and metro systems. This had precedents in systems like LA's, Philly's, and Newark's, where regular surface streetcars were routed into downtown subways to avoid street congestion.

In the 70s, the situation was reversed - planners realized that many downtowns had extensive downtown street grids that were under capacity, where lanes or entire streets could be given over to surface rail, avoiding the insane cost of downtown subways. In city neighborhoods and suburbs, freight-rail or freeway ROWs could be used to ensure high speeds, which were necessary in order to reach suburban destinations in reasonable amounts of time. These systems would have varying degrees of grade separation depending on the desired train speed and the levels of road traffic at each crossing.

Of course, because light-rail is just a way to describe some form of hybrid, it's impossible to define what IS and is NOT light rail. This is why Houston is building a "light-rail" system that's really just a big streetcar network, and why Honolulu is building a "light-rail" system that's really just a metro with short trains.