Country singer Merle Haggard and his four-bus entourage took a detour yesterday. On the road from Calhoun, Ga., to Warwick, R.I., they stopped by Amtrak headquarters to rent a train.

You may hear the whistle blow a thousand miles. The train will start in Bakersfield, Calif. (Haggard's birthplace) and stop at Willie Nelson's Farm Aid benefit in Champaign, Ill. The Sept. 22 Farm Aid concert aims to let country music stars help U.S. farmers in the way rock stars at the Live Aid concert raised money for famished African nations. Nelson, Johnny Cash, Waylon Jennings, Kenny Rogers and Alabama are among the artists pledged to perform at the event.

And as far as the 48-year-old Haggard is concerned, "If the train breaks down, we'll get a tractor."

Yesterday, the gaunt Haggard, wearing cowboy boots, met with Amtrak executives; James C. Rooney, deputy adminstrator of the Federal Railroad Administration; and Deputy Transportation Secretary James Burnley.

Burnley said he was there at President Reagan's request, adding that Reagan is "sympathetic and concerned" about the farmers' problems. Critics of administration farm policies have questioned that concern, but Haggard declined to join in. The administration, he said, has "a lot of things on their mind. They have blessed us with their approval." Haggard said his mission is with the American public: "What the public can do is help the farmer," he said.

What Haggard has already done is donate $10,000 to the Farm Crisis Committee. He also has written a new song, "Amber Waves of Grain," addressing the farmer's plight with this unequivocal message:

If the amber waves of grain should disappear

And there were no wheat or barley anywhere

Would we buy our bread and butter from the Toyota man?

Would an Idaho spud be stamped 'Made in Japan'?

The train's route and schedule is still to be worked out, though the singer plans to leave Bakersfield on Sept. 15. Amtrak Executive Vice President T.P. Hackney Jr. said that "as far as we are concerned, from what we have talked about today, it's a go." Amtrak is helping to expedite the route.

Haggard's music has always been populist and topical, notably in such songs as "Okie From Muskogee" and "The Fightin' Side of Me." He is also, according to his manager, Marc Oswald, an ardent train buff. Oswald said Haggard hopes to get financial help from outside sponsors but is prepared to pay for the train himself.

Farmers, celebrities and those scheduled to perform at the Farm Aid concert will be invited to jump aboard the 15-car train, The American, as it will be called. Cash and Nelson, said Oswald, already have agreed to participate. And President and Mrs. Reagan have received invitations.