Please read the third paragraph in my offering regarding Wikipedia, above. Yes,there is an operating dual gauge line in the world.
All the "odd-balls" in the US are transit, with no current need for compatability.
Sidelight: The Pennsylvania, and friends, arranged for legislation in their home state mandating a broader gauge for traction lines so that they couldn't get into interchange freight business. Still there in both ends of the state.
The real mystery is BART's 5' 0". If they really wanted better riding, they should have gone out to 6 or 7 feet. As it was, they significantly increased tie costs, car and rail delivery costs, and their track contractor's equipment costs. The advantage was that they "nearly" inherited their contractors' equipment.