There are a number of tier 4 compliant prime movers, using various control technologies, and SCR, available, or claiming upgradability for locomotives. Many appear to be very reliable and more fuel efficient. Tier 3 engines are well proven, more reliable, and today are over 90% cleaner than the locomotive fleet average emissions (primarily NOX, SOX, and particulates) was in 1993.

Cleaner diesel fuel is one of the key problems (garbage in=garbage out) and also presents a serious cost issue. Higher quality low/no sulfur and very low aromatic content diesel is expensive, well over twice as expensive on a gallon/btu mileage equivalent basis as CNG.

Natural gas (CNG) tier 3 and tier 4 compliant engines with very high reliability are increasingly replacing diesel truck engines in many "fixed base" trucking fleet operations. A network of "over the road" CNG truck fueling station are now being constructed in a several long distance trucking corridors.

CNG does work for RR's and has been tested. Large scale implementation issues include fueling infrastructure. fuel capacity, and crash worthiness issues for on-board storage.