Most of the transport literature only looks at mode alternatives in competition with each other, rather than exploring the potential for cooperation. This paper examines this possibility by making the case for aircraft and high speed train (HST) substitution under conditions of intermodal integration. In the model suggested, airlines use railway services as additional spokes in their network of services from a hub airport to complement and substitute for existing aircraft services. Airlines benefit from the slots that are freed and they can support mode substitution. Society gains from the social and economic benefits of better integrated transport services at a lower environmental cost. This paper examines this model of integration at Heathrow airport against the background of UK air transport policy and assesses the benefits and limitations of it. The conclusion reached is that some railway infrastructure should also be seen as part of the air transport infrastructure. © 2006 Elsevier Ltd. All rights reserved.