ELI5: Explain Like I'm 5

Track gauge

Track gauge is the distance between the two rails on a train track. This distance is important because it helps us to know what size trains can fit on the tracks, and it is especially helpful to keep trains from running off of the tracks. The most common type of track gauge is called Standard Gauge, which is 4 feet 8.5 inches. That means that when you look at a track, the distance between the two rails is 4 feet, 8 and a half inches. Other sizes of tracks, like those used by smaller trains, may be different measurements, but the Standard Gauge is usually used for most trains.