Abstract: The invention relates to an apparatus and a procedure for mounting a bearing wheel journal box (6) in a supporting structure. The supporting structure consists of a supporter side plate (4) having a seating (21) for the journal box (6). The seating has an opening delimited by corners (23,25). On the periphery of the journal box (6) there is an area (12) of reduced thickness, in the region of which the outer measure (S) of the journal box is smaller than the distance (W) between the corners (23,25) of the seating (21), while at other points the outer measure (D) of the journal box is larger than said distance (W). The journal box (6) is inserted into the seating (21) by holding the reduced-thickness portion (12) against the edge (23) of the opening and then turned into a desired position in the seating (21).