A self-contained gate assembly unit for ready attachment to an outlet formed in a hopper-type railroad car allows distribution of ballast in the car to the roadbed below. With the car having outlets positioned over the rails on which the car is supported, an inner or outer plate-like door, slidably carried in guides formed in the unit, may be raised to deposit ballast on either or both sides of either or both rails. Each door is formed with a lower offset lip portion which facilitates door movement in the unit guides.