Not all of the Class I railroads disclose train length information to investors (all are publicly traded or part of a publicly traded parent company). And among those that do, some do so by number of cars, others by overall length. Industry insiders say trains of 8,000 feet, or about 1.5 miles, are common, and anything longer is somewhat atypical.

But each of the seven Class I railroads — the largest freight carriers operating in the United States — has its own guidelines on making up trains into their final configurations.

And the Federal Railroad Administration, the nation’s train-safety regulator, doesn’t collect train-length data on a routine basis. Precise details on train length are part of accident investigations, spokeswoman Tiffany Lindemann said, such as after a derailment. But notably, there are no rules on length. It seems freight trains can be about as long as they want.

“Train length data is not collected by FRA regularly, and there are not regulations over the length of Class I freight trains,” Lindemann said.

What is known is that length has been increasing, albeit slowly, in some freight categories on an average basis, and on a trial basis in others.