Simplify the following text for a Grade 7 native English-speaking person.
Original: Apart from a few trains converted to work with the Victoria line 's 1967 Stock , or the Bakerloo 1972 Mk2 stock , all the Mk1 1972 Stock has been withdrawn .
Simplified: The London Underground 1972 stock is a fleet of trains used on the London Underground . The 1972 Stock was ordered to make up the shortfall in trains on the Northern line 's 1959 Tube Stock fleet .