Icahn in the filing said he spent about $40 million to buy the stake, which he has been building for about a month. The investor called Lake Oswego, Ore.-based Greenbrier "undervalued."

This is a business Icahn knows well. The investor is chairman of Greenbrier rival American Railcar Industries Inc. and owns 55.6% of the company.

American Railcar and Greenbrier have similar market capitalizations but Greenbrier is much larger in terms of sales, generating revenue of $443.5 million in the quarter ending Aug. 31, compared to American Railcar's $168.2 million in sales during the quarter ending Sept. 30.

Greenbrier on Nov. 1 warned investors that storm clouds were on the horizon. Company CEO William A. Furman in a statement said that he expects "less business viability" during the company's fiscal 2013 as a result of global economic and geopolitical uncertainty.

Company management estimate 11,500 to 13,000 deliveries in its fiscal 2013, down from 15,000 deliveries in the recently completed fiscal year. But Greenbrier is trying to push higher-margin cars including tank cars for the energy industry, which it says are in high demand. Tank cars are a specialty of American Railcar.

Icahn does not mention consolidation in the filing, but analysts have long predicted the investor could use his ownership of American Railcar to push dealmaking in the sector.

The investor was interested in such a deal in the past. Icahn in 2008, according to regulatory filings, acquired a stake in Greenbrier and approached the company about combining with St. Louis-based American Railcar, but nothing became of those overtures.

Icahn eventually exited that investment in Greenbrier, saying a merger was not possible due to some undefined unresolved issues.