Over the past year, transportation stocks have lagged other shares. Since last October, for example, the Dow Jones Transportation Index has lost 5.1%, while the S&P 500 index has gained 13.25%.

But not all stocks in the transport group have tracked the index. Railroad shares, for example, have outperformed both the sector and the overall market, with the S&P Supercomposite Railroad Index (a sub-index of the S&P Composite 1,500 index) rising by 17.3% over the period. That compares to, say, the S&P Supercomposite Trucking Index, which has dropped by 6.6%

Among the reasons for the relative strength in railroad shares: interest from value investors like Warren Buffett, and the fact that rising oil prices don't hurt this segment as much as other, more fuel-dependent industries.

Still, some might argue that at this point, much of the news, whether good or bad, is probably factored into prices. If you combine that with the fact that the railroad sector is back to long-term resistance levels relative to its trucking company counterpart, that suggests it might be a good idea to sell the former and buy the latter.

Otherwise, given a worrisome economic outlook and the relative underperformance of the transportation sector generally, it could be time for those who've been riding the rails to jump off the train -- before it runs out of track.

Michael Panzner is a 25-year veteran of the global stock, bond, and currency markets and the author of Financial Armageddon: Protecting Your Future from Four Impending Catastrophes and The New Laws of the Stock Market Jungle.