Once you have progressed enough through missions to unlock all the areas, you may see two stops marked in orange in the middle of Old Town. These are Cathedral stops, and to unlock them you must have at least 6 CNG buses in your fleet.
The fist set of missions that open up when you unlock the last area involve you having at least 4 CNG buses, and depending on your performance leading up to this point you may already have some CNG buses already.
Just keep chipping away at things and when you have enough money, buy the Iveco Urbanway 12 for €330,000. This is the first and cheapest CNG bus and is pretty nice to drive as well.
Once you have 6 CNG, the last stops unlock (without any notification or fan fare, I must add) and the achievement will pop.