What is it called?

That is called a Jif Lox, very common in the freight industry years ago. Roadway Express, Hyman Freightways, many other carriers used them. Makes a single axle tractor into a tandem so a driver could pull a heavy long box one direction, use the Jif to pull a set of doubles on the return trip. The original fifth wheel on the tractor would slide forward, right behind the cab, and the fifth whell on the converter dolly/jif would be moved forward some and pinned into place. The ones I have seen all had spring ride, as the frame rails were modified and did not have room for air suspension. The pic above shows an air ride suspension, however. This not what I had referred to about going into Alberta. A gooseneck type of short wheelbased unit with another fifth wheel in between the tractor and a heavy haul trailer to carry more weight in the combination since a tractor with lifting axle or axles is not allowed in Alberta. Hope you can understand my explaination.