In 1989, the TTC began using buses fuelled by compressed natural gas (CNG). Supported by subsidies from senior governments, the TTC used CNG buses to replace its trolley bus fleet. CNG buses were serviced at the Wilson Yard which had a special CNG fueling station. Because of safety concerns about CNG fuel tanks on the bus roof and low overhead clearances, these buses were banned from interior terminals. Also, the savings of using natural gas over diesel fuel was not as great as expected. The TTC converted some CNG buses to diesel.