The airline industry was hit hard by the recession—2011 had fewer takeoffs than any year since 2002. Airports in cities like Pittsburgh, Cincinnati, and Oakland are feeling the effects of that contraction, leaving one-time regional hubs and smaller airports with vacant and underused terminals.

A report on airport building reuse commissioned last year by the Transportation Research Board found enplanements were down more than 60 percent in St. Louis over the last decade. Growing interest in regional rail transit could place further pressure on smaller airports to get creative with their extra space, especially as they face costly demolition bills and shrinking revenue.