Get a look on the track topology on openrailwaymap:
https://www.openrailwaymap.org/?style=gauge&lat=62.774837258...
replies(4):
https://www.openrailwaymap.org/?style=gauge&lat=62.774837258...
It makes more sense for islands such as Ireland to retain their old gauge.
Probably more to do with the fact that the first train in Spain was build in Cuba
bast copy of the Subercase report I could find: [1] https://www.agrupament.cat/documents/Informe%20Subercase.pdf