The photo showing dual gauge is cool., much better idea than truck swap.
India has some cool dual gauge lines they have 3 rails one is common to both and there is two on the otherside they have meter guage, wide gauge, narrow gauge and standard gauge the most I saw in one place was a quad rail with meter gauge, 5’6”, and narrow gauge
I think they still have to do the swap once they get across the border.
I have looked at Google maps to try to figure out what they do at the French- Spanish border. It appears that Spain does have some variable axle cars. And the high-speed network in Spain has been built standard gauge instead of the national standard.