lst = ['Bus04_00_00_IB_pts_Line', 'Bus04_00_00_OB_pts_Line',
    'Bus15_00_00_IB_pts_Line', 'Bus15_00_00_OB_pts_Line']
[(key, list(val)) for key, val in itertools.groupby(lst, lambda x: x[0:5])]
Group similar items in a master list and create new lists based on grouped items