Ask a Capitol Hill lawmaker why he or she opposes or supports a piece of legislation, investigation or policy and you are likely to get back an answer based on principle.

They support regulation or deregulation, they will say, or free trade, or fair trade, or some such. But if you dig just below the surface, you usually find a less theoretical reason, such as a political contributor, an old sore, or jobs in jeopardy.

The vast majority of lawmakers, however, generally skate over that vast area of politics that is all about mutual back-scratching, IOUs and scores yet to be settled.

That is not the case with Rep. Barney Frank (D-Mass.), who is notoriously prickly, markedly bright and — this goes with the first two qualities — brutally candid. This last quality can be refreshing (depending on where you happen to be standing) in a federal capital saturated with spin and humbug.

Frank’s candor was on robust display recently, as The Hill’s Jessica Holzer reported Wednesday, over efforts by CSX Corp., a big railroad operator, to stir national security concerns about a proxy fight with two British hedge funds.

Because railroads are part of the national infrastructure, and because CSX is the country’s biggest transporter of military equipment, the company wants lawmakers to prevent the hedge funds securing five of the 12 seats on its board.

Sens. Evan Bayh (D-Ind.), Tom Carper (D-Del.), Sherrod Brown (D-Ohio), Mel Martinez (R-Fla.), Jim Bunning (R-Ky.) and Robert Menendez (D-N.J.) wrote to Treasury Secretary Henry Paulson seeking an interagency review, saying the funds’ investments in CSX “lack transparency, leaving the ownership and control of critical U.S. infrastructure in the hands of unknown investors.”

We make no comment on the merits of this argument, except to say that the British are not the most worrisome of possible foreign investors — perhaps, actually, the least.

Our interest, rather, is in Frank’s response to CSX, which was dismissive. “Why,” he asked, “do I want to protect their right to screw my state?” No wordy concerns about national security there.

Frank explained that CSX “has been so incorrigible with us over the commuter rail, I’m disinclined to do anything for them.” His reference is to a dispute between CSX and Massachusetts over liability for deaths in train accidents.

Interesting though these details may be, it is Frank’s message that is fascinating — not for being unusual, but for being unglossed: Mess with me and don’t expect any favors in the future.

What a neat encapsulation of why corporations and interest groups spend so much time and money kissing up to members of Congress.