The economy is stuck in neutral, but the natural gas transportation industry is moving full speed ahead by addressing two main problems inhibiting the industry.

More stations
In July, Chesapeake Energy (NYSE: CHK) committed $1 billion to finance the construction of a natural gas refueling station network. Natural gas station provider Clean Energy Fuels (Nasdaq: CLNE) will receive $150 million of Chesapeake's $1 billion investment, with additional financing apparently on the way.  

Three investment firms recently agreed to provide an additional $150 million of financial backing. In total, Clean Energy Fuels has obtained a $300 million purse over the last two months, which will bankroll a network of approximately 300 natural gas stations. 

The additional 300 stations brings Clean Energy Fuel's total natural gas fueling station count to more than 500, making it the dominant natural gas station provider in the industry. The company now owns and operates nearly half the total number of natural gas stations in the U.S.

Cheaper engines
Historically, the incremental cost to convert a heavy-duty truck from diesel to natural gas was approximately $60,000 to $70,000. That vast expense has suppressed sales in the past, but the future could look much different.

Cummins (NYSE: CMI) and Westport Innovations (Nasdaq: WPRT) recently rolled out a natural gas engine that sells for roughly half the price. The substantial decrease in conversion costs could catalyze the industry by substantially reducing the payback period for companies interested in converting to natural gas. 

The bottom line
An expanding natural gas refueling infrastructure, coupled with the decreasing incremental cost to convert, could jump-start the natural gas vehicle industry. Investors should keep an eye on these stocks as this long-term-growth story continues to unfold.