Simplify the following text for a Grade 9 native English-speaking person.
Original: Metronet Rail was the brand of a public-private partnership within the London Underground group that was responsible for the maintenance , renewal , and upgrade of the infrastructure on nine London Underground lines .
Simplified: Metronet Rail was the brand of a partnership within the London Underground group . The group was responsible for the maintenance and upgrade of the infrastructure on nine London Underground lines .