A new exchange-traded fund that invests in "progressive" transportation companies – ranging from makers of electric-vehicle batteries and drive systems to bicycle makers and railroad and shipping giants – will soon add its name to the a growing roster of ETFs focused on green businesses.

Invesco PowerShares is set to open its Global Progressive Transportation ETF to investors on Sept. 18. The fund will track the Global Energy Efficient Transport Index, or HAUL, from Encinitas, Calif.-based WilderShares, the creator of several green-energy exchange-traded indices that have seen assets grow with the rising price of oil and increasing interest in greener investment opportunities.

But are there enough public green-transportation companies to make up a whole index?

HAUL has addressed the issue by capturing two themes: the "innovative companies that are just on the horizon with new technologies" that people would expect to see in the index and lower-tech transportation, which is more efficient than regular gas-guzzling cars, WilderShares CEO Robert Wilder said.

The first category includes companies like Ener1 (AMEX: HEV), a lithium battery and fuel-cell maker, and Quantum Fuel Systems Technologies Worldwide Inc. (NSDQ: QTWW), which makes the drive train in Fisker Automotive Inc. hybrid electric cars.

But most of the index is comprised of companies involved in transportation methods "that are simply more efficient to begin with," Wilder said. That includes "everything from scooters and bicycles," including Vespa maker Piaggio SpA and bicycle makers Shimano Inc., Giant Manufacturing Co. Ltd. and Merida Industry Co., to developers of subway and passenger train systems and shipping and logistics companies like International Shipholding Corp. and OceanFreight Inc.

And then there are the other companies in that second category, such as railroad giants like CSX Corp. and Union Pacific Corp. and oil and natural-gas pipeline and shipping companies like Enbridge Inc. and Williams Cos., that may not strike the environmentally conscious investor as particularly clean or green.

"That's something we talked about very early on. People kept saying, ‘How can you include a traditional railroad [company] – they're so dirty,' " Wilder said.

"The most energy-efficient way to move something from point A to point B is railroads," he said. "Could you have an energy-efficient transportation index that didn't have rail? To me, that would raise eyebrows."

Including tried-and-true rail, shipping and fossil-fuel companies in a "green" transportation portfolio could be a good way to avoid placing too many bets on an electric and alternative-fuels industry that's still in its infancy, noted Ying Wu, senior analyst with Lux Research Inc. in New York City.

While she sees long-term promise for companies working in the plug-in hybrid and pure-electric vehicle space, "I think that market is three to five years in the future," Wu said. "I'd take more of a wait-and-see attitude, rather than putting the money in there now."

She also suggested adding Toyota to the index for its leadership position in the hybrid car market, and concentrating natural gas-fueled vehicle investments in companies working on building up the distribution network that would fuel them.

Wilder noted that the HAUL index, now less than a month old, will continue to grow and refine its methodology.

"In time, we'll be able to pick the greenest of the railroads," as well as add more companies in the electric, hydrogen and gaseous fueled vehicle space that meet the fund's criteria, such as a $200 million market capitalization or greater, he said.

WilderShares' other green technology indices and matching exchange-traded funds include the WilderHill Clean Energy index, traded under the symbol "PBW;" the WilderHill New Energy Global Innovation index, traded under the symbol "PBD;" the WilderHill Progressive Energy index, which tracks companies involved in making fossil fuels cleaner and is traded under the symbol "PUW;" and the Cleantech Index, which concentrates on companies involved in cleaner energy, water and industrial production and is traded under the symbol "PZD."