Here is the answer from my son's father in law as promised.
You can take this as gospel and not some internet story as John is
very experienced in this field and is an engineer.

Good to hear from you. Natural gas has been quite widely used on vehicles,especially buses, and especially in US and Canada for about the the last 20 years. When I was with (Company name removed, Bazz) we built quite a number but they were never very popular with the operators because of high maintenance costs and poor reliability. We used purpose-built Cummins natural gas engines powered by compressed natural gas (CNG). This is simple but the tanks take up a lot of volume and weight. You can use liquid natural gas (LNG) but it requires low temperatures, thermos flask type tanks and tends to distill off the methane ( the main constituent) leaving behind the lower liquifying temp gases e.g. propane ;butane etc. These tend to accumulate in liquid form at the bottom of the tank and if not purged off from time to time get into the engine and damage it.

This is less of a problem with the CNG but everything depends really on the purity of the natural gas. The higher the the methane content ie >90% the more reliable the engine and hence the vehicle. In UK it can be as low as 81%. At these sort of concentrations the engine will run but the various sensors in the system become bunged up or fail altogether after a time. Also there are 2 other practical problems


1. Methane is lighter than air. So if you keep vehicles in an enclosed spaces you have to have a specially designed roof that automatically vents any natural gas to atmosphere otherwise methane concentrations build up in roof spaces and form explosive mixtures. Can be exciting!
2. Filling the vehicle tank can be slow. If you pump too fast the liquified gas becomes hot and vapourises so that it becomes difficult to fill the tank completely full or indeed half full.