The US transportation sector, which many see as a proxy indicator of the economy's health, has retreated 3.1 percent from its September 14 record, hinting to some analysts that the longest bull market on record has entered its late stages.

Railways, freight carriers, and package deliverers get less attention than heavy-hitting momentum stocks like Apple Inc and Amazon.com, but the sector could be showing cracks in what analysts and the US Federal Reserve characterize as a robust economy.

Several constituents of the Dow Jones Transportation Average (DJT) have provided disappointing guidance in recent months. As the third-quarter reporting season approaches, investors will watch to gauge whether trade, fuel, and dollar risks are affecting the sector's bottom line.

"If the transports were to break down further from these levels, if you saw them declining another two or three percent in the near future, you would call that a bearish non-confirmation," said Michael O'Rourke, chief market strategist at JonesTrading in Greenwich, Connecticut.

Delta Air Lines Inc is due to report on October 9, a week after lackluster forecasts from the company and its peer United Continental pulled US airline stocks lower. The bulk of the companies in the DJT are expected to post results in the latter half of October.

Although a preliminary deal to replace the North American Free Trade Agreement (NAFTA) has boosted railway stocks, looming tariffs threaten to increase the cost of transporting goods and services, further testing other DJT constituents.

Brent Crude prices have risen nearly 26 percent since the beginning of the year and energy analysts see the trend continuing well into 2019.

"In the near-term, we've seen prices increase as a result of Iranian sanctions reducing the supply of crude oil to the market," said Andrew Lipow, a consultant at Lipow Oil Associates in Houston. "I expect that over the next year, the price in transportation fuel is going to be increasing."

The climbing dollar could also pressure transportation companies as US goods grow less affordable to foreign consumers, which might result in fewer shipments abroad.

"The net effect is that the strong dollar could impact the transportation index over the course of the next 12 months," said Bernard Baumohl, managing director, and chief global economist at the Economic Outlook Group in Princeton, New Jersey.