The line is operated by Renfe Operadora and runs through the Guadarrama Mountains from Cercedilla, Madrid to Cotos Pass [es], Segovia.

Construction began in 1919 and the section connecting Cercedilla on the Madrid-Segovia Line with Navacerrada opened on 12 July 1923 as the Guadarrama Electric Railway.

In 1990, the Guadarrama Electric Railway was incorporated into RENFE's commuter rail network, Cercanías Madrid, and renamed Line C-9.

They were replaced by second hand SECN stock, nicknamed Navals, from other metre gauge lines, principally from the Ferrocarill de la Loma.

From here, the railway line crosses the drainage divide of the Guadarrama Mountains and enters Valsaín Valley in Segovia Province.

After a gradual climb, the line reaches its terminus of Cotos Station [es] with a peak elevation of 1,819 m (5,968 ft) above sea level.