This paper revisits the concept of air/high-speed rail (HSR) integration in the specific case of congested airports, in which airport slots for (super) short-haul flights are freed by replacing them with high-speed trains. Freed slots are then likely allocated to longer flights, which leads to an increase in GHG emissions induced by flights from/to the airport into question. Such an unexpected effect is investigated through the case of Frankfurt Airport, where the HSR infrastructure was designed to connect smoothly with the airport. The ex post investigation isolates the time window during which airport capacity is stable. It confirms the increase in aviation climate impact. This illustrates that air/HSR integration is not always a relevant solution to curb the impact of aviation on climate change.