sentences:  London Overground operated eight two-car Class 172s on the Gospel Oak to Barking line, replacing the old stock.The units were originally meant to be in service from February 2010, but due to production issues and then what were thought to be exhaust problems (which proved to be a false alarm), the trains entered service only in July 2010, and at reduced capacity due to staff training problems.By December 2010 the whole fleet was in service, allowing the Class 150s to be cascaded to First Great Western and other operators.