A manway assembly for a railroad tank car features a nozzle defining a central opening and having a sidewall, a cover, a hinge attaching the cover to the nozzle, and a seal assembly to seal the cover to the nozzle. The seal assembly may include a circumferential seal and a circumferential seal channel. The circumferential seal may include a top portion, a bottom portion opposite the top portion, and a side portion between the top portion and the bottom portion. An underside edge portion of the cover may be provided with the circumferential seal channel. The circumferential seal may be positioned within the circumferential seal channel. The circumferential seal may have a C-shaped cross section and may be positioned in the circumferential seal channel such that the top portion of the seal is seated against a top wall of the circumferential seal channel and the bottom portion of the seal is seated against a circumferential ledge of the nozzle.