Complex sentence: These mainly worked the semi-fast Harrow and Uxbridge services and ran on the East London line as modified four-car sets, and allowed the Circle Stock to be replaced by 5-car trains of O and P Stock.
Split into simpler sentences: These mainly worked the semi-fast Harrow and Uxbridge services, although they also ran on the East London line as modified four-car sets. This transfer allowed the Circle Stock to be replaced by 5-car trains of O and P Stock.