The 3-axle technology is clearly visible. These were unique cars. Thanks for the photos.

For 3-axles, I can only refer to this photo and commentary: https://transphoto.org/photo/1557280/#2901165
It is a historically interesting subject.

A can refer everyone to the excellent 2-volume book series on 3-axle cars by Henry Elsner of Philadelphia:
https://www.robertgavora.com/pages/books...ory-and

I knew the author personally, his specialty was unique subjects in streetcar history, such as 3-axle cars, early XXth century low-floor ('stepless') cars, etc. He even attempted a project on RVZ-6 cars...

Then you can certainly congratulate the author for this book: a unique work. It is also (and only!) thanks to him that I understood the purpose of this type of vehicle, or rather, the technology used.
It is a huge puzzle, with pieces scattered all over the world over a very long period of time, but the author has managed to put it all together.

Funny, I followed your link _after_ I posted my comment. And I found the same book reference in your text ) Another testament that Henry's work is the only true source on the subject.