Yes, the scientific methods suggest that 99.9% of all systems and almost all of the very best highest performing systems have fares. 'Light rail' in the US are barley even transportation systems. At best they are property value plays by cities. Running a single line in a city full of cars isn't really interesting data.
And in other places they had light year that gets paid that had effects that were much the same.
Once you are talking about an actual working transportation system and not the occasional line here and there. Where it is actually real then there is a clear scientific question. Lets assume 500M to operate the system:
Should you A: Invest 500M in service improvements every year
Should you B: Invest 500M in giving away free transport
And in other places they had light year that gets paid that had effects that were much the same.
Once you are talking about an actual working transportation system and not the occasional line here and there. Where it is actually real then there is a clear scientific question. Lets assume 500M to operate the system:
Should you A: Invest 500M in service improvements every year
Should you B: Invest 500M in giving away free transport
And the science on this is perfectly clear.