Drive assembly for a door, gate or barrier, having a rail or track, a trolley for connection to the door, gate or barrier, mountable to run along the rail or track, and a longitudinal drive having a chain or belt connectable in a closed loop arrangement around or within the rail or track. The trolley is provided with an engagement mechanism for selective engagement between the trolley and the longitudinal drive mechanism, whereby the engagement mechanism includes an engagement member arranged for selective movement in a direction substantially transverse to the longitudinal direction of the longitudinal drive and in a direction less than 45° from the plane of the loop of the longitudinal drive. The engagement member includes or cooperates with a cam part arranged for selective rotation about the engagement member, and a camming cooperation occurs between the cam part and the trolley body for converting rotation of the cam part about the engagement member into the required movement of the engagement member.