sentences:  The units were leased by London Overground Rail Operations (LOROL), the London Overground concession operator, from a rolling-stock leasing company rather than purchased directly by Transport for London.The success of London Overground's operation of the Gospel Oak to Barking Line meant that the 2-car Class 172s eventually proved insufficient in terms of capacity.Initially, there were plans to replace them with 3- or 4-car DMUs.