Okay kiddo, imagine you have two train tracks that run side-by-side. The distance between the two tracks is called the rail gauge. Rail gauge is really important because it determines the size of the trains that can run on those tracks.
Think about it like this: if the two tracks were really close together, only small trains would be able to fit on them. But if the two tracks were farther apart, bigger trains with more passengers or cargo could run on them.
Different parts of the world have different rail gauges that are best suited for their needs. For example, in the United States, the standard rail gauge is 4 feet 8.5 inches (which is about the same as the width of a normal car), while in Europe, the standard rail gauge is a bit wider at 4 feet 11 inches.
It's important that rail gauges are consistent throughout a country or region because trains can't just magically adjust themselves to fit the different gauges. So, engineers have to make sure that the tracks are built to the right size and trains are designed to fit those tracks.
So, rail gauge is kind of like a rule that tells everyone how wide the train tracks should be so that trains can run smoothly and safely.