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: The Marylebone sets , which were later transferred to Bletchley but remained on Chiltern Line duties , were declared surplus after the introduction of Class 165s , from 1991 onwards .