Commercial air services in Europe have experienced a spectacular growth in the last 15 years. From 1985 – 2000, the main European airports doubled their operations. Moreover, in the last 20 years, the number of regional airlines grew from 32 in 1980 to 78 in 1999. This growth has resulted in a continuous increment of delays in air services. In 1986, 12.5 percent of air flights were delayed 15 or more minutes; the figure nearly doubled to 23 percent in 1998. In summer 1999, the average delay was more than 26 minutes. It is, therefore, not surprising that the congestion costs borne by European airlines have increased from $2,600 million in 1991 to $4,900 million in 1999. This article presents results of studies undertaken for the Ministry of Public Works in Spain on the effect of new railway investments in reducing slot number needs at Madrid Airport. A total of 54,000 slots are compared to other studies carried out in France (40,000 slots in Paris-Charles de Gaulle) and Germany (20,000 slots in Frankfurt).