They use Standard gauge, 4 foot 8 1/2 inches, like us, as do most of Europe apart from Spain. That is why trains can run freely through the Chunnel and travel across most of Europe.
I thought they'd outlawed the use of imperial measurements in the EU, so surely that should be 2.2479m gauge?
I'm an Imperial minded Lady so let the Europeans call it what it likes!
...and anyway standard gauge there is
1,435 millimetres (4' 8 1/2")
As a sideline note I have always chuckled over the Spanish Canfranc International Railway Station in the Pyrenees, that was an extensive complex built to speedily handle through traffic into France and back. However a mistake was made somewhere during the planning. It was suddenly realised that the Spanish track gauge of 5 ft 5.8 inches (1,672 millimetres) did not match the French main line gauge of 4' 8 1/2" (1,435 millimetres)!! Ooooops!
Up until 1970 all that meant that lengthy stops took place with an interchange of passengers between Spanish trains and French trains!
Also, as an interesting historical fact, many Nazis escaped to Spain as the Third Reich collapsed taking their looted wealth!