All good things come to an end sooner or later. I was high on the railroads earlier this year, and Canadian National (NYSE:CNI) was one of my favorites. Now, though, it seems the bull run in rails may be approaching the end of the line.

For the third quarter, Canadian National reported revenue growth of just 6% -- the first single-digit performance in the three quarters I've followed the company for The Motley Fool. Margins improved again, so operating income and net income growth both exceeded the top line, growing more than 12% and nearly 19% respectively. But again, that growth is starting to slow relative to prior quarters.

Most of the top-line growth came from rate increases. Revenue ton miles increased only a bit more than 1%, and carloads hauled were basically flat with the year-ago period. Within those numbers, coal, grain, and forest products were strong, while petroleum, chemicals, autos, and metals were less so. Not surprisingly, intermodal revenue was still strong, rising 10% on healthy activity at the port of Vancouver.

Looking ahead, I think it's easier to make a bearish case than a bullish one for the rails. Fuel costs are still high, and the company has less than 20% of its anticipated 2006 demand hedged -- a situation that could cost it hundreds of millions of dollars if prices stay high. What's more, if the economy begins to soften as some think it might, I'm not sure why there would be ongoing strength in shipping rates. If businesses produce fewer widgets, they need fewer raw materials, and that could mean less demand for the commodity rail traffic that is CN's specialty.

On the brighter side, the company is still making hay while the sun shines, and cash flow has improved notably over last year. What's more, rail transit is a very cost-effective shipping option, and it seems as though more customers are moving from road to rail when circumstances allow.

I'll certainly be paying attention to upcoming reports from the likes of Burlington NorthernSanta Fe (NYSE:BNI), Canadian Pacific (NYSE:CP), CSX (NYSE:CSX), and UnionPacific (NYSE:UNP) with an eye toward rate trends and forward guidance on freight expectations for 2006. I'm admittedly nervous about Canadian National today, but I still believe it's the best-run major railroad on this continent -- perhaps in the world. That's good news for patient investors who can ignore short-term economic cycles and concentrate on the long track ahead.

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Fool contributor Stephen Simpson has no financial interest in any stocks mentioned (that means he's neither long nor short the shares).