TfL Rail had originally planned to replace the existing Class 360 units with Class 345s, but due to delays with the signalling system in the Heathrow Airport tunnels and late delivery of Class 345 units, a contingency plan has been implemented. The Class 360s will continue to be used between London Paddington and Heathrow, supplemented by two Class 345 trains per hour running between London Paddington and the extended bay platform at Hayes & Harlington, until the Class 360s can be fully replaced. The Class 360s were finally retired in September 2020