What: Kansas City Southern's (NYSE:KSU) stock fell 17.9% last month. That capped a bad year, in which the railroad's shares fell around 38%. But it was an early December conference that likely led to the monthly decline.

So what: On Dec. 2, Kansas City Southern's CFO Michael Upchurch presented at a Credit Suisse conference. The news wasn't good. Upchurch started by recounting that October and November volume numbers were, year over year, continuing a year-long downtrend. Everyone knew that. But he added that he expected the full quarter to see a "high single-digit" revenue decline.

What was interesting were the sources of the company's weakness. Although there was general strength, including in transporting oil, Upchurch noted coal and intermodal as particular weak spots. Coal was no surprise, since natural gas has been displacing the fuel. A great example of the issue can be seen at Cloud Peak Energy, which has seen sales volumes fall each year since 2011. This year, the thermal coal miner is expecting a particularly large year-over-year volume decline of as much as 12%. And Cloud Peak is one of the better-positioned coal miners, so Kansas City Southern is feeling the effect of this broader trend.

Weakness in intermodal, however, is a bit more concerning. Customers have been using alternative shipping options, notably trucks, because of quality issues on Kansas City Southern's system. Although the railroad believes it can win that volume back, it has to prove itself anew to customers. Upchurch, however, also noted that the peak season wasn't as good as expected because of high inventory levels at its customers. That's a bad sign for future shipments until those inventories are worked down. So there are really two headwinds blowing in intermodal right now.

And that led to an overall negative feel, which was likely what investors reacted to. Although the statement came early in the Q&A, Upchurch summed things up when he suggested that 2016 is going to start on a weak note. Cue investors getting nervous about owning economically sensitive railroads, particularly one working back from service issues.

Now what: Railroads are a vital link for transporting goods around the country and world. And Kansas City Southern plays an important role in that, notably its cross-border operations. So, from a long-term perspective, this railroad has a good business. With the shares well off their late-2014 highs, investors should be taking a look. That said, the suggestion of a weak start to 2016, coupled with quality issues on the company's rails, should probably keep conservative investors on the sidelines for now.