The buses aren’t generally free though: https://www.mbta.com/fares/bus-fares
They are free on only 3 lines (plus SL1 leaving the airport) https://www.boston.gov/news/mayor-wu-takes-steps-expand-fare...
Free buses should not be considered in any city. Spend that money on more frequent routes. People will not use a free bus that only comes once an hour, or doesn't stop close to them. Until your city is covered with transit coming every 5 minutes you need to work on that not cost.
Not even the best transit cities have transit every 5 minutes all over the city. There is a lot of room to improve in them all
[1] https://www.boston.gov/news/new-steps-reduce-vehicle-emissio...