The Consejería de Obras Públicas, via the Urban Planning and Transport Department, realized that it was necessary for the Metro to reach eight outlying districts whose residents will benefit most from the Expansion Plan. The new stations were projected to be used by more than 111 million passengers each year. The new extension also reach two areas of great importance, not only for Madrid but for the whole of Spain -The Juan Carlos I Exhibition Center and Madrid - Barajas International Airport, which was linked by an extension of the new Line 8.

Likewise, there it was plannet to link the airport with the city center via a Metro line through the Mar de Cristal station (see plan), whose detail design is finished and that will start in June 1999.

The Metro is estimated to carry 42,000 travelers each day between the city center and Barajas Airport.

Every soft ground tunnel is a difficult job, but Madrid's varied geological characteristics make it even more difficult to build underground transport due to many subterranean water currents as well as pronounced slopes that make any Metro project a complicated and challenging engineering problem. Spain's leading tunnelling engineers and construction conglomerates have joined forces in this project, and they have succeeded in building more than twice the length of Lisbon Metro Extension, Athens Metro Extension or London's Jubilee Line Extension in half the time, and at less than one fifth of the cost.

Construction was carried out using six EPB tunnelling machines -four of 9.38 meters diameter baptised "La Adelantada" (Mitsubishi-NFM), "La Chata'' (Mitsubishi-NFM), "Almudena" (Herrenknecht) and "Paloma" (Herrenknecht), one of 7.4 meters diameter called "Cibeles" (Lovat) and the smallest one, the "Puerta del Sol" (Herrenknecht) of 6.7 meters diameter. In addition, certain sections were constructed using either the traditional Madrid Method or cut and cover methods with diaphragm walls.

All the tunnels are two-ways except in Line 8, where the 2 smaller EPB machines have been used and tunnel diameter is smaller.

The principal aim has placed great emphasis on safety and quality. Cost and time frame were given less priority. The whole expansion plan was monitored by some six thousand geodesic sensors sending back data on any ground movement or slippage to a central location.

Today, with all lines commissioned, the high safety standards have been maintained and corrective measures taken when any ground movement exceeds given safety parameters.

Thirty seven new stations

Trains

Lines opened

Metro users have already had the opportunity to familiarize themselves with the new trains and stations. All sections of the Plan have already been opened, totaling 25.3 km and 22 stations. The first, in 26th December 1996, was the Lago-Plaza de España stretch through the Príncipe Pío station, which serves as a link with the Spanish mainline rail system. This line was built under the River Manzanares using the Lovat EPB machine and the Classical Madrid method.

On 22nd January 1998 the Gregorio Marañón station was inaugurated as well as part of the new Line 10. This line joins the northern and south-eastern parts of Madrid (Fuencarral- Aluche), considerably reducing travel time and making it easier to change to other lines. On 13th March 1998, the first part of Line 7, between Avenida de América and Gregorio Marañón, linking it up with the recently completed Line 10, was commissioned. On 27th April 1998 the new extension to Line 4 from Esperanza to Mar de Cristal stations was commissioned. Here, in Mar de Cristal, starts new Line 8 to the Juan Carlos I Exhibition Centre, that was commissioned in 24th June 1998.

On 16th October 1998 a further extension to Line 7, from Gregorio Marañón station to Canal station was opened, with a new station in Line 2, improving connection between lines. A month later, on 16th November 1998, new Line 11 was opened, from Plaza Elíptica to Pan Bendito stations. On 1st December 1998 the new extension to Line 9, from Pavones to Puerta de Arganda, was commissioned, with another 4 new stations. On 15th December 1998 the final extension to Line 4, from Mar de Cristal to Parque de Santa María stations, was opened, and on 12th February 1999 a further extension to Line 7, from Canal station to Valdezarza, was commissioned. The link with Barajas Airport was commissioned as June 14, 1999.

The 38 km underground extension, with 34 new stations (also underground), has had a final cost of 1,361 million $USA. That makes a unit cost of 36.19 million $USA per kilometer of completed line. Including the new rolling stock, 226 cars valued at $247 million, the final cost is 42.76 million $USA per kilometer of underground line.

Detail design of Line 10 started in August 1995. Design of Line 4 and 8-10 connection started in September to November 1995. Design of Line 7 extension started in January 1996, Line 1 and Line 8 to Exhibition Centre in May 1996 and Line 8 to the Airport in July 1997.