Turned-out to be a "solution" at all O48: (with the over-under at O60min with 4% grades)
Perhaps with an On30 narrow-gauge loop on a mountain at 8" in height. Shown as Kato Unitrack at 17" radius. Access hole in the middle of the mountain: 2'x2'. An O36 FasTrack loop could be substituted. Three trains running without intervention at the junction or crossings.
On the O gauge: O42 and smaller runs everywhere; O54 runs the over-under.