On long-distance trains in Spain and night trains crossing from Spain into France, the arriving locomotive stops just short of the gauge changer, uncouples and moves into a short siding out of the way. Gravity then moves the train through the gauge changer at a controlled low speed.

Spanish trains change gauge using an ingenious piece of engineering known as a Variable Gauge Axle and a physical "gauge changer" track. Historically, Spain's traditional rail network was built using the "Iberian Gauge" (1,668 mm), while the rest of Europe and Spain's newer high-speed (AVE) lines use the "Standard Gauge" (1,435 mm). To allow trains to travel seamlessly between these two networks without passengers having to switch trains, companies like Talgo and CAF developed specialized wheelsets. As the train enters a gauge-changing facility (typically a small shed on the tracks), it slows down to about 15 km/h. The weight of the train is temporarily taken off the axles by side rails, while a series of mechanical "locks" on the wheels are released. The tracks then gradually taper inward or outward, physically sliding the wheels along the axle to the new width. Once in the correct position, the wheels are locked back into place, and the train continues its journey. This process takes only a few minutes and is almost imperceptible to passengers, allowing Spain to run high-speed trains that can transition from modern 300 km/h lines to older regional tracks in a single trip.