Regarding Natural Gas as a motor fuel, a friend who drives transit bus in Phoenix says they are having BIG problems with the high - pressure fuel lines bursting. Apparently the stuff ( LNG or CNG I don't recall which ) is very corrosive, and is eating through the fuel lines, requiring replacement with stainless steel lines. He also comfirms the limited range as being an issue. This can be solved in the railroad environment through the use of fuel tenders ( see above shot of Canadian National units w/ tender ); however this amount of CNG / LNG could be a rolling bomb in case of a derailment. No First Responder would go anywhere near it.
I agree with Rich Hunn : As the demand for CNG / LNG rises, so too will the price. This will likely cancel out any financial advantages for using it, leaving only the emissions advantage. As others have noted, a Tier 4 Engine running Low - Sulfer Diesel is just as clean.