ATSF 520977, Atchison, Topeka & Santa Fe Athearn 50' plug door side-braced boxcar, stock#1344 released in 1971 (Tony Cook's site)
This car number belonged to the series 520750-521447 of class Bx-114 (insulated box, XMLI, total 700 cars), built by the railroad Topeka shop in 1967, equipped with 10' lever type Youngstown plug-doors. ("Santa Fe Box Cars, the Shock Control Era 1954-1995" p192-193, 260)