What: Norfolk Southern Corporation's (NYSE:NSC) shares fell 12.8% last month. That comes after a nearly 20% rise in November driven by a failed takeover attempt by Canada's Canadian Pacific Railway (NYSE:CP). What goes up must come down, but there's more going on at Norfolk Southern.

So what: The big news at this railway recently has been its seemingly successful attempt to fend off Canadian Pacific's roughly $30 billion acquisition bid. Investors sent the shares higher on news of the proposed deal, but Norfolk wanted no part of it and said no. So that's one reason the shares were weak in December: They were simply retracing earlier gains.

But that's not the only reason. At a Credit Suisse conference on Dec. 2, Norfolk Southern's chief marketing officer, Alan Shaw, recounted a number of industry- and company-specific headwinds that exist apart from the Canadian Pacific issue, reminding investors that all is not well in railroad land. For example, he explained that "50% of our revenue is tied to commodity prices," which has led to a negative volume mix. Since commodity prices remain in the doldrums, this issue isn't going away any time soon.

The other big negative was in fuel surcharges. Essentially oil prices fell, so customers didn't have to pay extra to ship their goods. This was a surprisingly large piece of a year-over-year decline in revenues. Although Shaw expects that to anniversary out of the numbers next year, it doesn't change the fact that oil prices remain low today. Which helps explain why the railway is looking to alter how it charges for fuel expenses.

In the end, Shaw recounted a number of negatives but presented a generally upbeat outlook for Norfolk Southern. Investors, however, seem to have fixated on the negatives, since several big-picture issues aren't likely to change in the near term. Add in the November price bump for an takeover attempt that doesn't seem likely to happen, and you get a December price decline.

Now what: Norfolk Southern is one of a small number of large North American railroads. That said, there are headwinds today that should rightly concern conservative investors. But as the takeover attempt suggests, there's plenty of value here. Indeed, Norfolk is in prime position to benefit from trade trends over the long term. With the shares off late-2014 highs, Norfolk is worth a deep dive for most investors, though it wouldn't be surprising to see operating weakness persist into at least the early part of 2016.