boxcar
Synonym discussion
Sense: noun/1
'Boxcar', 'freight car', 'railcar', and 'goods wagon' are close, but they are not exact matches. 'Freight car' is the broad American term for any train car used to move goods. 'Boxcar' is narrower: it means a closed freight car with side doors, so it contrasts with open cars or tank cars. 'Railcar' is wider still and can also refer to other kinds of railway vehicles, so it is less exact when you mean this specific cargo car. 'Goods wagon' is the usual British term, especially in rail writing. If sacks are stored inside a covered cargo car, 'boxcar' fits best; if you mean cargo vehicles in general, 'freight cars' is safer.
Etymology
'Boxcar' is a native English compound: 'box' points to the vehicle's enclosed, box-like shape, and 'car' names the railway vehicle. The noun appeared in nineteenth-century North American rail use for a covered freight car. Later, English developed an adjective sense meaning 'very large' or 'very high', probably from the big numbers painted on the sides of boxcars.