As it happens, this very topic came up during the club “Skype session” last week. There was, apparently, once a well-known layout of this type on the U.K. exhibition circuit - this is NOT recent! It was a product of the 1940s-50s era when scratch building was probably at its peak.
O Gauge 2-rail hadn’t really caught on in U.K. at the time, and O sized insulated wheelsets simply weren’t available at the time.
The crux of it was that the dual gauge section comprised a centre rail, all as previously described, using the then-new proprietary 16.5mm gauge wheels, O and OO Gauges. All running rail was the same size.
Narrow gauge switches were conventional in construction, but the track plan was constructed so that there were no O Gauge switches within the dual-gauge section, thereby avoiding the problem.
All junctions where the narrow gauge crossed the centre rail, were wired as “live frog” crossings - ie, bounded by rail breaks forming isolated blocks, so that the whole area took the polarity of the live rail.
It would be possible to construct O gauge, dual gauge switches with a “fixed frog” - the moving section being confined to the section between the narrow gauge rails - but 2 rail track building techniques weren’t sufficiently developed at the time.
That was all that could be remembered by the group, but it seemed plenty!
