In railroad terminology, a gondola is an open-top type of rolling stock

Rolling stock

Rolling stock comprises all the vehicles that move on a railway. It usually includes both powered and unpowered vehicles, for example locomotives, railroad cars, coaches and wagons...

 that is used for carrying loose bulk materials. Because of its low side walls, gondolas are used to carry either very dense material, such as steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 plates or coils, or bulky items such as prefabricated pieces of rail track.
Before the opening of the Baltimore & Ohio Railroad and the Chesapeake & Ohio Canal in Harpers Ferry, Virginia (now West Virginia), considerable amounts of coal

Coal

Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

 were carried via the Potomac River.