Depends on fuel availability. Diesel is available everywhere. CNG has limited availability. In my county, we do have propane powered busses.
CNG and propane have much better emissions profiles, and vehicle lifetime and compressed tank lifetime are a good match for transit, as opposed to personal vehicles where when the compressed fuel tank ages out, the otherwise servicable vehicle turns into a pumpkin.
However, CNG ends up being expensive and may not save much versus diesel... The natural gas is usually not expensive, but compression requires a lot of energy input which is expensive.
