The Dow Jones Transportation Average (DJT) was falling in afternoon trading on Monday, a bearish sign for the broader markets.

The DJT, a price-weighted average of 20 transportation stocks traded in the U.S., is studied to confirm the state of the U.S economy, especially by advocates of Dow Theory.

This theory says that as the industrials boost output and production, the transports should be shipping more of their products, confirming upward trends in both the DJT and the DJI.

A divergence from these correlating moves indicates a potential reversal of the trend and economic weakness.

On Monday, shares of several railroad stocks, in particular, were sinking in afternoon trading, following a bearish note from Morgan Stanley regarding rail operator CSX (CSX) , which was downgraded to "Underweight." 

Shares of CSX were falling over 3%, while shares of its counterpart Norfolk Southern (NSC) were sinking over 1%. Other rails in the red were Union Pacific (UNP) , along with Kanas City Southern (KSU) , lower nearly 1% and almost 2%, respectively.

Also lower were shares of trucking and transportation company JB Hunt Transport Services (JBHT) , down over 2%.

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