WASHINGTON — William Withuhn, a licensed locomotive engineer who, during his 27 years as transportation curator at the Smithsonian National Museum of American History, could have been called America’s official train collector, died June 29 at his home in Burson, Calif. He was 75.

The cause was heart disease .

Mr. Withuhn was an expert on all modes of transportation — planes, trains, and automobiles — and helped the American History Museum acquire many major items, including Richard Petty’s No. 43 Pontiac stock car, which he drove for his 200th and final NASCAR win.

He was also a sports-car enthusiast who had flown about 200 combat missions as a navigator in the Vietnam War, but his greatest fascination was with trains. He grew up in the railroad town of Modesto, Calif., and while in his 20s received his engineer’s certification.

He later operated short-line railroads in Virginia, Maryland, Delaware, and New York and became a railroad historian, preservationist and advocate in Congress before turning his love of transportation into a career at the Smithsonian.

‘‘Americans ride the trains in their hearts,’’ he wrote in a 2002 essay for Newsday. ‘‘Trains built America, physically interlacing a huge geography and splintered political regions into a national union. . . . We became the most mobile nation in the late 19th century, with trains.’’