Beyond that I haven't heard much of it because as I understand it, there isn't much free transit anywhere big.
My claim was based on skimming an article that said fares ranged from about 10 to 50% of funding and I speculated that most probably tended toward 10% since most are small, and small ones depend on federal funding, special purpose taxes (Georgia has a lot of SPLOST[0] projects), and partnerships.
Smaller systems suffer from low ridership and might see more benefit from dropping fares. A full bus and an empty bus cost about as much to run, but a system of full buses is easier to bolster and improve because more people will support it.
And I don't see why this would be connected to behavior on buses and trains. Where is this research? At least for Seattle since everyone wants to focus on it for some reason. News articles for a single city are not research.
[0] https://en.wikipedia.org/wiki/Special-purpose_local-option_s...