This is actually quite a significant technical achievement - for example, a similar project in Japan failed.

Japanese Railways wanted to build a train that can run at full speed (~300 km/h) on the standard gauge (1435 mm) regular Shinkansen lines but also use the narrow gauge (1067 mm) existing lines at slower speed. Those older lines would not have to be rebuilt for the Shinkansen standard & there would still be significant time savings:

https://en.wikipedia.org/wiki/Gauge_Change_Train

This failed to produce a viable train, resulting in falling back to track rebuilds or using relay trains that connect directly from Shinkansen to the local rail line on the same platform.

They don't seem to say why it didn't work.

I wonder if it was not just the change in gauge, but tolerances as well.

I got to see "Dr. Yellow" running on the shinkansen line and it checks everything out.

https://en.wikipedia.org/wiki/Doctor_Yellow

I wonder if the slower-speed lines have looser tolerances.

Then maybe running the gauge-change train on the slower lines might kill the train's tolerances before it moves back to the super smooth high-speed lines.

Swiss trains can, but while stopped.

There's a station on the main line that loads full sized cars with tanks on them onto little bougies that take them up into the mountains for training.

Swiss train can do it in motion, the post above is about the MOB train that can go from Montreux (meter gauge) to Interlaken (standard gauge).