Simplify the following text for a Grade 15 native English-speaking person.
Original: The British Rail Class 115 diesel multiple units were 41 high density sets which operated the outer-suburban services from Marylebone usually to destinations such as High Wycombe , Aylesbury and Banbury which are on the Chiltern Main Line and Great Central Main Line ( now the London to Aylesbury Line ) .
Simplified: These 41 high density sets operated the outer-suburban services from Marylebone usually to destinations such as High Wycombe , Aylesbury and Banbury which are on the Chiltern Main Line and Great Central Main Line ( now the London to Aylesbury Line ) .