The old Madrid Burgos line had a collapsed tunnel near Somosierra.
When a landslide caused the derailment of the train between Gumiel de Izán and Fontioso in 2010, ADIF, the state-owned railway infrastructure company in Spain had a very straight solution: Abandon the Somosierra tunnel all together and replace it with a bus service.
The decision makers at ADIF thought, after all, there were not many passengers using the train service. Only 6 at the time of the collapse. So there was this bus which took passengers from the departure station and stopping at every railway station until the destination station.