I've been noticing for years that some lorries have a retractable axle that can be carried and isn't rolling on the road. A bit of research shows that this has many different names but that "lift axle" got the best results. There is some info here. It looks like the application is mostly manual rather than automatic. The axle isn't driven - it runs freely. The decision to deploy is a balance between the fuel and tyre-wear saved by reduction in rolling friction, and the need to spread the axle loads more to meet legal limits.
