Light rail is the cross of a lot of things. Sometimes is just a new name for a streetcar system. It is also interurban like. Other times it is a scaled down version of a heavy rail, rapid transit system. Othertimes it is using 50 year old cars, other times it is the newest part of a fleet.
The beginings of light rail started in Frankfurt Germany in 1968 when Siemens develeped a rail a car called the U2. It was an articulated car about 70 ft long, had low platform loading was was powered by overhead wires. It was not intended to be a streetcar, nor an interurban, nor a rapid transit car, yet it could do a little of all of these.
Edmonton, AB was the first city in North America to get a light rail line in 1978. Even though the city is not as large as others, this did not stop them from digging a subway tunnel trough dowtown Edmonton. The line continues north east, adjacent to a mainline railroad. It would have been possible to build a full rapid transit line as there is no street running. The line itself uses high platforms throughout.
San Diego, CA was the next city to get light rail in 1981. Just as planning was being done on the line, Southern Pacific had a a major washout on its line to San Diego. It was not interested in restoring service and sold the line a a very low price. A couple of sidings were added, some tracks were layed in the streets of downtown San Diego, wires where strug up. With that a very low budget light rail line was created. There was such a fear that the system would fail, the absolute minimun infastruction was built. Unlike Edmonton, low platforms are used.
Calgary, AB would be the next light rail system in 1982. It is similar to San Diego in that it uses street running in downtown Calgary but with high platforms.
Siemens would sell many more light rail cars in. All three of the first systems have continued to grow. Portland, Sacramento, Denver, St. Louis, Pittsburg, Los Angeles, Salt Lake City all run or will run Siemens light rail cars.
Even with flexibility of Light Rail, it is not a universal solution. Even though most light rail systems use standard guage tracks, it is very rare for them to operate along side main line railroads. Commuter rail systems such as Metrolink fill this market.