Salt Lake City's commuter-train system could be called "really light rail," if the state's Division of Air Quality succeeds in having it powered by natural gas.

Ran Macdonald, state environmental engineer, has asked Salt Lake's light-rail steering committee to support use of liquefied natural gas, or LNG, locomotives instead of diesels.LNG locomotives would create less air pollution, require less maintenance and have a fuel cost comparable to diesel, Macdonald notes.

Salt Lake Clean Cities - a coalition of 65 public and private entities pushing for use of alternative fuels - is backing the proposal. The Utah Transit Authority, which will run light rail, already has been using 10 natural gas-powered buses and vans since the early 1980s.

UTA officials say they haven't been impressed with the performance of those alternative-fuel vehicles. Spokeswoman Coralie Alder said the vehicles didn't work as well in higher altitudes and lacked the power of diesel engines.

"I understand that the technology's getting much better. Certainly the door is open for alternative vehicles. We're still considering it," Alder added.

Two forms of natural gas can be used as alternative fuels - compressed and liquefied.

UTA has been using compressed, which comes in a gaseous form, takes a lot of tank space and doesn't work well in anything other than light duty equipment.

LNG is natural gas that has its temperature lowered to 260 degrees below zero Fahrenheit, at which time it turns to liquid and is easier to carry in tankloads. Studies show it also burns cleaner than diesel and provides more power than other alternative fuels.

But LNG still has its problems. Because it must be so cold, equipment for conversion, transportation and storage is costly and scarce. A current large scale liquefaction process at a gas plant costs about $4 million, Macdonald said.

Air quality officials have their eye on two LNG locomotives currently running on Burlington Northern freight lines in Glendive, Mont.

Burlington Northern wants to reconvert the engines to diesel only but would be willing to trade for equivalent diesel locomotives.

Macdonald estimated that LNG locomotives can be purchased for about $750,000 each as used equipment, compared to the $3 million listed for new equipment.