As anyone knows who has read last year's batting records or gone out to the airport to greet a major league team, the .400 hitter and the train are equally extinct. They are for everybody but the good people of Minnesota, where the Twins have a .400 hitter who owes his career to a train and a trainman. Second Baseman Rod Carew, who hit .400 last week to raise his season's average to .392, was born on a train in Panama and was first recommended to the Twins by a railroad detective named Herb Stein. "Herb mentioned a friend of his had seen a good prospect playing in New York, so we decided to give the player a tryout," recalls Cal Griffith, the Twins' owner. "After he planted a few balls in the seats, that was enough. We signed him." The prospect was Carew, who joined the Twins in 1967 and became Rookie of the Year. In mid-August last season his average was .309, but then he started trying to hit home runs and dropped off to .273. "I thought about that during the winter," says the left-handed hitter. "I knew some adjustments had to be made. I switched from a 34-to a 36-ounce bat, opened my stance and forgot about hitting homers. All I do now is think base hit and I don't care how I do it, even if I bunt it, drag it, chop it or slash it." In one game Carew had two bunt singles and an inside-the-park home run. Last week he hit two homers in one game and later taught Red Sox ace Jim Lonborg how much his new attitude and stance have helped. "We used to get him out with a high, inside fastball," says Lonborg. "I threw him one tonight and he pulled it down the right-field line for a double. That hit went like a bullet." Or, maybe, a speeding locomotive.