Back in the seventies several cities tried out the n.g. motors. At the time they were very underpowered. Things may have changed now with technology but i don't know.
Here in upstate NY, the municipal buses are powered by natural gas, and I believe that NYSEG (ny state electric and gas) has most of their fleet on n.g. Oddly enough, NYSEG is now owned by a company in Spain.