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G is usually 1:22.5 in scale and 45mm track, representing metre gauge. There are other scales from 1:22 - 1:32 running on the same track.
16mm is on both 32mm and 45 mm, representing 2 and 3 ft gauge and is 1:19 Scale.
In essence, they are close enough for anyone with metre gauge and 3 ft to run both together....