open freight car; gondola car
Technical term in rail transport for a freight car without a roof, used for bulk cargo like coal or gravel.
Coal is loaded onto the open freight car.
有蓋貨車 is a covered freight car (boxcar), while 無蓋貨車 is an open freight car without a roof.
Compound of 無蓋 (no lid/cover) and 貨車 (freight car).