For more than a century, followers of Dow Theory have looked to transportation stocks as the great bellwether of broad market movement.
For the past five years, though, the practice hasn't held up particularly well.
Nonetheless, it's been a somewhat happy divergence in that transportation stocks have far outperformed their industrial brethren. By mid-August 2014, transportation stocks have posted gains of 265 percent since the March 2009 low, compared to the 165 percent gain of the widely followed Dow 30.
Technical strategists who follow the ebb and flow of chart patterns are, for once, at a loss as to what has caused the differing performances. Some suggest the transports are simply not a good way to look at a technologically evolving economy anymore.
Others believe the stocks comprising the index are too sensitive to give an accurate reading. Some even suggest the move is all part of market distortions that have accompanied unprecedented central bank market interventions.
"The transports do still matter, but they don't matter as much as they did in 1910," said Richard Ross, global technical strategist at Auerbach Grayson. "The industrials are actually lagging, and the transports are surging. Maybe people need to ask themselves whether the strength in the transports is actually reflecting what's going on in the broader economy. Is there something happening to make people chase the transports?"
Questions like the ones Grayson raise are significant going forward because the U.S. and global economies are only on the cusp of what is likely to be a paradigm shift in the engines of growth.
As demands change, transportation demands will change with them. Companies like Amazon already are testing out same-day service, and others are likely to follow suit.
From an investor standpoint, that's going to raise questions about whether transportation is an accurate barometer of economic progress or whether technology will emerge as being even more important.
David Blitzer, who makes his living watching sector indexes as managing director and chairman of the Index Committee at S&P Dow Jones Indices, believes transports still matter.
"If you look through the companies in the index, you cover a huge range of moving goods, from raw materials to finished products around the country and to some extent around the world," he said. "It's a list that gives you a very strong sense of what the sensitive portion of the economy is doing."
There's something to his sentiment.
The 20-stock index contains such ubiquitous names as FedEx, whose earnings are usually considered a key read on economic activity, through a swath of airlines, including Southwest and Delta, to some more obscure names like rail and marine fleet operator GATX.
Many analysts would be hard-pressed to give a good accounting of the names in the transport index, though it often is cited in discussions of whether the index is "confirming" a move higher in the much more closely followed industrials.
"It's a surprisingly good sampling of who moves stuff around the United States and what it takes to move stuff around the United States and around the world," Blitzer said.
What moves the index, though, isn't always so easy to tie to the broader economy.
Airline stocks have led the surge higher for the transports, which as an index had gained more than 10 percent by the second week of August.
Southwest had posted a 52 percent gain, Delta was up 33 percent, and JetBlue rocketed 38 percent higher amid a buying fervor that seemed to have little to do with bigger growth trends. Those gains trumped weak performances by FedEx and rail freight firm Kansas City Southern—both companies that would seem to be more in tune with economic growth.
Abigail Doolittle, who studies technical trends at Peak Theories Research, has been watching the transports and industrials diverge sharply over the past two years, and it's a move that has left her befuddled.
"The move in the transports is more shocking to me than any of the other global indices," she said. "It's the first time I've ever thought of market manipulation. I used to look at the transports first. Now it's kind of an afterthought. They have lost that sort of edge for me."
Investors looking for the transports as guides to market movement should go elsewhere, Doolittle added.
"Transports as a true 'tell' for what's to come in the market and economy—that's just not been the case for the past two years," she said. "The 10-year (Treasury) yield has remained a true tell. The Russell 2000 definitely has been more accurate than the transports."
Still, few expect any major changes ahead for the index. While companies may come and go, the base of rail, air and ground travel will remain.
Blitzer chuckled when asked if he thought drones could change the index but did say he could foresee a satellite-launching company get there in the next quarter century.
"It's a pretty wide selection of items that, without these companies, nothing would move other than electronics and the Internet," he said. "I think the general mix is likely to remain."
—By Jeff Cox, finance editor, CNBC.com