The Dow Jones Industrial Average and the Dow Jones Transportation Average moved closer together the week ending Sept. 28th, helping to close a gap that's opened this year as the DJIA has surged 9% and the DJTA has fallen 2.5%. Both indices were down this week, but the DJTA fell only slightly, 0.4%, while the DJIA fell more than 1%.

According to the Dow Theory, both indices must move upward in tandem to support the idea that a broad recovery is taking place. Unmatched gains by one index are thought to be unsustainable. As regional Federal Reserve banks have been sequentially announcing declining or stagnant manufacturing activity, industrials have suffered.

Earlier this week, heavy-equipment manufacturer Caterpillar (NYSE: CAT) cut earnings estimates for the next three years, projecting slower economic growth than the Federal Reserve expects. The stock is down 6.2% for the week, helping to drag the DJIA down.

With diversified operations covering many construction projects across the world, Caterpillar is seen as a bellwether company, and its gloomy assessment of global growth sent other equipment manufacturers down as well, with mining equipment supplier Joy Global (NYSE: JOY) down by 5.6%.

Of course, Caterpillar may be priced for pessimism, and at the end of this week it sold for less than 8 times forward earnings. The Motley Fool has put together a premium research report on the opportunities and risks facing Caterpillar. Get your copy today to help you decide whether today's prices are a good entry point.

The Dow Jones Transportation Average was supported by only mild declines in the railroad sector, as well as a particularly strong performance by truckers. J.B. Hunt (Nasdaq: JBHT), the largest American trucker by market capitalization, was up 2.4% for the week after Moody's upgraded its debt, citing a strong financial position. Moody's also noted that J.B. Hunt has excellent exposure to intermodal trucking, the most promising growth segment of the freight industry.

C.H. Robinson Worldwide (Nasdaq: CHRW) was also strong, up 1.7% for the week after announcing it would acquire freight-forwarder Phoenix International. C.H. Robinson doesn't own trucks but instead acts as a third-party logistics provider, primarily through domestic truck brokerage. The Phoenix acquisition will boost the company's exposure to ocean and air freight forwarding, which currently accounts for only 7% of C.H. Robinson's revenue.

The Dow Jones Transportation Index's strongest performer was regional airline holding company Alaska Air Group (NYSE: ALK), up 4% for the week after management declared a new share-buyback program on Wednesday. The $250 million initiative, If executed, would repurchase around 10% of Alaska Air's total market capitalization. The company has targeted December 2014 to complete the program and will fund the repurchase out of cash on hand.

While stock market pullbacks aren't necessarily fun for investors, I'm continuing to watch how transportation providers and manufacturers perform relative to each other. A sustainable economic recovery will require broad gains in both sectors, so as these indices move together I'm more confident about global growth.

One important area to watch for both will be energy prices. Transportation companies prefer cheap energy prices to offset their fuel costs, while manufacturers like Joy and Caterpillar perform better when energy prices are high, since this drives investment in energy production. Natural gas might be experiencing rock-bottom prices now, but one Motley Fool analyst expects prices to start rising by 2014. Luckily for investors like you, we've identified one stock you need to own for the natural gas revolution. This report is free, but it's available for only a limited time, so get your copy today.