Good day everybody!
I have spent quite a while googling away at the following question.   In 2009 I bought a Atchison Topeka Santa Fe train set with steel rails,  Locomotive is #49. 
I would like to purchase a set of tracks to make a circle wider than the current 4 foot circle. I thought the existing track was G gauge but the distance between rails for G gauge is 45mm and the existing track is about 49mm. 
Is the existing track a proprietary gauge?

Thanks!