Any municipality, county, or combination of municipality and county, incorporated or existing under the laws of Tennessee, additionally has the power and authority to pay unto public or private transportation companies for the local carriage of passengers a reasonable subsidy of their operation, the amount of the subsidy to be wholly in the discretion of the governing body of any such municipality, county, or combination of municipality and county, to ensure continued operation of a public transportation system in any such municipality, county or intrastate or interstate metropolitan area.