This can be used in conjunction with #13593 to change the placement of vehicles on a track.

Example use:

        veh->remaining_distance += distanceToMove;
        veh->SetUpdateFlag(VEHICLE_UPDATE_FLAG_SINGLE_CAR);
        veh->UpdateTrackMotion(nullptr);
        veh->ClearUpdateFlag(VEHICLE_UPDATE_FLAG_SINGLE_CAR);

remaining_distance is internal distance units. Issue at present is that velocity of the vehicle is also applied. Unsure if that is useful or not. If not can easily fix that by changing how the override of the update code works.