Among the Dow averages, most investors focus on the Dow Jones Industrials (^DJI -1.29%), because they encompass the widest range of industries. But the Dow Jones Transportation Average (DJINDICES: DJT) is also important, and lately, its strength has been more impressive than the tepid gains from the Dow Jones Industrials. Let's take a look at why strength in marine-delivery company Kirby (KEX -1.44%) led the Transports higher, while railroad giant CSX (CSX -1.21%) held the average back, and how prospects for both could have an impact on the Dow.

Source: Kirby.

Kirby's strong performance came after the company reported solid earnings Wednesday night, lifting the stock up 6% for the week. Kirby's first-quarter results included record earnings per share, with net income rising almost 10% on a 5.4% jump in revenue. Despite some negative impact from winter weather, Kirby said that its marine markets have seen high demand that have helped the company gain pricing strength. Inland and coastal utilization rates were in the 90% to 95% range, with strengthening operating margins helping the marine division's profitability. Although barges aren't the sexiest part of the transportation business, they play a vital role in moving goods, and strength from Kirby means that the customers that rely on Kirby to get their products to market are looking healthier than in the past.

Source: CSX.

Meanwhile, CSX saw modest declines of around 1% after one of its trains derailed in Virginia. The train, which was carrying oil, caught on fire, and investors fear that the long-term impact of the derailment might be a more critical look at the practice of having railroads transport crude oil and other energy products. CSX and its railroad peers have relied on the need for oil producers to transport crude from hard-to-reach shale plays, and any regulation could crush what has been a vital source of revenue at a time when other commodities have seen prices plunge.

From a broader perspective, railroads and marine transportation play a key role in economic activity. Although they're not as familiar to average consumers as airlines or overnight-shipping companies, the companies in the Dow Jones Industrials rely on them as a vital link between production and consumption. If the Transports keep faring well overall, they should help the Dow Jones Industrials keep performing well also -- and potentially send the Dow to new record highs.