Since the 1980s, a lot of American cities have built new light rail lines in an attempt to rekindle the golden age of streetcars. And many European cities have never given up on light rail.
These light rail lines are expensive to build and inflexible. There is a better way however: take out the rails and lay down some asphalt, then take the trainsets and add some rubber tires.
It's called Bus Rapid Transit. Just like light rail, it has a separate right-of-way, so it can bypass congested roads, but unlike trams, buses can leave BRT lanes and take to the streets.
The cost savings are especially spectacular in North American cities where light rail is often built by ripping up a lane of traffic to put in tracks. Bus Rapid Transit only requires separation barriers to keep traffic off.
And you can put these lines anywhere. In 1954, the Los Angeles Metropolitan Traffic Association suggested building busways underground!
So, it seems like it would've been best if cities had scrapped all their light rail lines and instead gone with BRT. The cost and flexibility advantages are tremendous.