def get_rail_z_extent(rail_part):
    """Get the dimension of `rail_part` along the z-direction.
    
    :param the_model: The meshed part 
    :type the_model: Part object (Abaqus)
    
    :returns: Dimension of `rail_part` along the z-direction.
    :rtype: float
    
    """
    
    rail_part_bb = rail_part.nodes.getBoundingBox()
    
    return rail_part_bb['high'][2] - rail_part_bb['low'][2]