On drives, a tag is behind a drive, pusher is in front of a drive. On trailer its a lift if it comes of the ground. Then there is the Axle Rose as mentioned. One of its own and on the Nightrain. How many know what a JIF tandem are?
It was a tag axle/frame/fifth wheel assembly. It attached to the fifth of a 2 axle tractor and turned it into a 3 axle.
I remember Matlack's attempt to open a small site in Bangor, ME. To take advantage of the intrastate higher weight limits they added a subframe just to the rear of center looking kinda like the one for the dolly legs and added an air lift axle