'Canfranc had been one of only three places the railway crossed the border. In 1936 Franco had closed the border and ordered the walling up of the train tunnel (now used as an underground laboratory for research into dark matter, which requires the sort of “cosmic silence” found deep within mountains). With petrol scarce and no more trains, the only way out was on foot.'

https://www.ft.com/content/45b4bafa-a218-4007-bbc2-2c68b6533dd1