The following commentary comes from an independent investor or market observer as part of TheStreet's guest contributor program, which is separate from the company's news coverage.NEW YORK ( ETF Expert) -- You don't have to agree with every aspect of "Dow Theory" to appreciate one of its most venerable features. In particular, the Dow Jones Transportation Index should lead the way higher for the Dow Jones Industrial Average. Why has this particular axiom maintained a level of popularity for more than a century? In essence, transporters "take" materials to the industrial giants that "make" goods. It follows that if transportation companies are increasing their profits, their share prices should rise, thereafter blazing a trail for industrial producers.