Abstract: A vent-on-demand hatch cover for closing a hatch opening of a compartment in a railcar, and particularly a hopper car, wherein the car is provided with a coaming that defines the opening. The cover includes a peripheral opening for taking in the outside air and which is disposed adjacent the outer side of the coaming and an inside opening at the underside of the cover exposed to the compartment for connecting the cover to the compartment air. Passageways are provided between the two openings in order to allow air flow between the outside of the car and the inside of the car. A closure member is provided at the inside opening of the cover which is resiliently biased normally into closed position and which opens upon the formation of a vacuum within the compartment in order to allow unloading of the compartment.