It's easy to add more cars to the train.
It's not easy to add cars because you'd have to extend existing platforms to accommodate longer trains, which is usually expensive and time consuming.
This is not to discard the entire point about the value of light rail, but that particular argument is incorrect.
* Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!