The gauge (the distance between the rails) of the FWR track is 45mm. This fits a number of different scales.
Our 'home' scale is 16mm:1ft (1:19). In this scale the track is a nominal 3ft which doesn't really fit any English railways but some in the IOM and in continental Europe.
7/8th's scale (~1:14) results in slightly bigger models and some 'basic' garden railway loco's are of this scale, including one of ours.
"G Scale" is a bag of spanners. It's nominally 1:22.5, unless it's American where it's 1:20.5. Generally it's a variable scale so that things fit the 45mm track, regardless of the size of the original. So, ironically, the one scale that has "scale" in the name, is the one that isn't a scale.
G1 is where 45mm fits 'standard gauge' - the classic 4ft8" of the main line railways.
Over time I've run most scales on the FWR(s) except G1. We had a bit of a neighbourhood running session this afternoon and Steve brought along a G1 train (blipped) and we all were pleased to see it run fine on the track.
So, today we ran all scales simultaneously (I'd a few G-Scale trucks). Glad no-one was much of a perfectionist :-)