A door unit for rail vehicles includes one or two sliding doors, means for supporting and guiding each sliding door between a closed position and an open position, and means for controlling the movement of each door between a closed position and an open position. The supporting, guiding and controlling means are carried on a support unit which defines an independent module with the doors, assembled prior to being mounted on the vehicle structure. The support unit includes at least one frame connectable to the vehicle structure and supporting each door, and the respective guiding and controlling means. The frame includes an upper cross-member, a lower cross-member and two uprights connecting these cross-members between them, in such a way to form a perimeter frame that defines a doorway, and which surrounds the four sides of the door or doors, when they are in the closed position.