A variable gauge system allows railway vehicles in a train to travel across a break of gauge between two railway networks with different track gauges.

For through operation, a train must be equipped with special bogies holding variable gauge wheelsets containing a variable gauge axle (VGA). The gauge is altered by driving the train through a gauge changer or gauge changing facility. In effect, the track widens or narrows.

As the train passes through the gauge changer, the wheels are unlocked, are moved closer together, or further apart, and are then re-locked. Installed variable gauge systems exist within the internal network of Spain, and are installed on international links between Spain/France (Spanish train), Sweden/Finland (Swedish train), Poland/Lithuania (Polish train) and Poland/Ukraine (Polish train).

A system for changing gauge without the need to stop is in widespread use for passenger traffic in Spain, for services run on a mix of dedicated high-speed lines (using Standard gauge) and older lines (using Iberian gauge). Similar systems for freight traffic are still in their infancy, as the higher axle weight increases the technological challenge. Although several alternatives exist, including transferring freight, replacing individual wheels and axles, truck exchange, transporter flatcars or the simple transshipment of freight or passengers, they are impractical, thus a cheap and fast system for changing gauge would be beneficial for cross-border freight traffic.

Alternative names include Gauge Adjustable Wheelsets (GAW), Automatic Track Gauge Changeover System (ATGCS/TGCS), Rolling Stock Re-Gauging System (RSRS), Rail Gauge Adjustment System (RGAS), Shifting wheelset, Variable Gauge Rolling Truck, track gauge change and track change wheelset.