Fun fact from Wikipedia:

> Since the beginning of the 1990s new high-speed passenger lines in Spain have been built to the international standard gauge of 1,435 mm (4 ft 8+1⁄2 in), to allow these lines to link to the European high-speed network.

(...)

> In general the interface between the two gauges in Spain is dealt with by means of gauge-changing installations, which can adjust the gauge of appropriately designed wheelsets on the move.

I didn't know that!