Making public transportation free would reduce demand for oil/gas in cities and would lower prices people in rural areas would pay relative to it not being free.
Of course, this assumes that public transportation is only available in cities, but that's not true at all. Many European countries have extremely effective rural public transportation.
Oil is a global commodity so local demand reduce won't help much. Even if rural public transportation is good, I don't think rural people don't need a car, so they would complain about unbalanced grant.
The costs are negligible to every corp bailouts in the last 50 years or the cumex scandal. With the money corps extracted through illegal means with cumex we would have free transportation for the next 30 years.
Roads are vastly more expensive than transit per trip. If having the busses and trains full avoids yet another widening then taxes go down, and the rural + urban areas stop subsidizing the suburbs.
