Jim Cramer. video.

"Right now we have a roaring bull market in the railroads," said Jim Cramer. But how do you harness the runaway bull?

Rather than ride CSX , Kansas City Southern or any of the other railroads, Cramer suggested hopping on a different play.

"When the railroads are doing well, they order more rail cars," explained the Mad Money host.

And lately, they've ordered a lot.

In the fourth quarter, the industry received orders for 11,021 new railcars, when the analysts were only expecting 10,000.

Therefore – Cramer suggested playing the theme with rail car makers.

But not just any old rail car maker. He's most interested in companies that make tank cars. That's because of all new orders in the fourth quarter, tank cars made up 62% of that total.

(Tank cars are basically big tanks on wheels that are used to transport all sorts of liquids, which need to be handled with care—everything from crude oil to various types of chemicals to corn syrup.)