Railway track and railroad carriage gauge transfer
Gauge is the distance measured between the inside edges of two rails, or the outside edges of two wheels. There must be a match between wheels and track. Changes in railway track and railway car gauges may happen when moving across country borders, within countries, and also in mountainous regions where narrower gauges are more common.
There are different ways to accommodate gauge changes in railway tracks and railway carriages.
Laying extra rails is a way to handle different railway car gauge widths.
Here's an example diagram of twin gauge tracks:
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Here's an example of triple gauge tracks:
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Another way to deal with differing railway gauges is to make adjustments to the rail cars so they fit different track gauges. Engines do not usually have that ability and are swapped out at the gauge change transfer point, but there are two ways to adjust railway cars to fit different track gauges. Both involve changing the width between railway car wheels.
The wheels on a rail car are attached at each end of the car, and are called "bogies".
Some rail cars have detachable bogies, allowing swapping different wheel gauges onto the rail car. The cars can either be lifted off the track, or else placed on a section of track that can be lowered, which allows the bogies to be removed and replaced with other bogies to handle another gauge. The location where this is done, and the act, are both called a "bogie exchange".
A less common solution to the mixed gauge scenario is adjustable bogies, which still require removal from the track. Adjustable bogies don't have to be removed from the cars because they have a mechanism to change the width between their wheels.