We made a partially visible message on the wall here: '01/11/2017 Hiking Madrid'!
The tunnel is 3 km long and on a curve, so the light at the end of the tunnel soon disappears!
I occasionally walk with Hiking Madrid and today was the All Souls Day holiday walk called the Walk of the Day of the Dead. It followed this disused railway line UNDER La Sierra de La Cabrera.
But this tunnel hides a sad tale of forced labour under the Franco regime. The tunnel was dug by prisoners, many of them political prisoners, over time and the railway line was finally opened in 1968. It was never financially viable and the last recession killed it off. It closed in 2012. When I first started walking in the Sierra de La Cabrera in 2011, I saw the trains occasionally and I hadn't realised the line had closed until this walk along the line and through the tunnels was advertised!