Tandem axle with rear tag axle controlled from cab

the big reason they did that was mpg, one less rear gears to turn robbing hp. bleeding air off for better traction not so much. the problem is the drive breaks traction and it's to late to dump the air.

 

We use them in Europe all the time, average fuel savings over a 6x4 is 1mpg sometimes even more, depending on fifth wheel position they are actually better on snow and ice especially when empty, the thing I see with American trucks is that they are short behind the cab, on a rear lift ideally you need the fifth wheel forward , almost directly over the drive axle, this gives superior traction but also prevents you taking too much weight off the steers so you don't under steer , another thing euro trucks have is weight transfer, you can transfer some weight off the tag onto the drive without lifting, I had a rear steering tag before which was excellent especially the handling in slippery conditions
if you are buying new I'd spec a long enough wheelbase that you can have the fifth wheel pretty far forward

 

Cab air control is legal on axles with legal weight.

 

Schneider Nationals used to run from Freightliner Argosys with the same setup. Another added benefit is that raising the tag axle can give you a shorter turn radius for getting into tight spots. With a setup like that, you have to ensure that you have the right amount of air pressure going into it - too much places additional weight on your steer axle, as the single drive axle begins to act as a fulcrum, and the air pressure from the tag pushes upwards on the rear of the frame, which translates to more force put on the steer axle.
You see tags used quite a bit on vocational truck applications.. in addition to the allowed weight increase from having additional axles on the ground, you also gain an increase in permitted weight with the extended wheelbase when the tag is lowered. Which is also why you have dump trucks with the strong arm, and mixers with the Bridgemaster.