Covered Gondola? Someone explained one just like that before. My immediate thought was Covered Gondola, was right. Unless you mean something like the front half is box, the back is tank. That would be rather strange.
you probably saw a covered gondola though if it had a gon bottom and curved top-
they're used to transport rolls of steel and that's why the curved top
basically unrideable and i thought i was gonna havta ride one on huron central rr once but luckily thought better