L = ['Bus04_00_00_IB_pts_Line', 'Bus04_00_00_OB_pts_Line',
    'Bus15_00_00_IB_pts_Line', 'Bus15_00_00_OB_pts_Line']
d = collections.defaultdict(list)
for elem in L:
    d[elem.split('_')[0]].append(elem)
print(dict(d))
Group similar items in a master list and create new lists based on grouped items