By Cara Lombardo and Miriam Gottfried

A group of big buyout investors is considering a takeover bid for railroad operator Kansas City Southern that could be worth more than $21 billion and mark a big bet on U.S.-Mexico trade.

Blackstone Group Inc.'s infrastructure arm and Global Infrastructure Partners are together exploring a potential deal and speaking to banks including Citigroup Inc. about financing, according to people familiar with the matter.

There is no guarantee they will proceed with a formal offer or that Kansas City Southern would be receptive. Assembling the roughly $15 billion equity check that could be required for a deal of that size would pose significant hurdles for Blackstone and GIP, which are investing out of $14 billion and $22 billion infrastructure funds, respectively.

If there is a deal, it would be significant. Kansas City Southern had a market value of roughly $14 billion Friday afternoon and including debt, the value of a bid could exceed $21 billion, some of the people said.

The bid being discussed would likely include about $6.5 billion worth of debt financing, these people said.